diff --git a/data_acq_and_processing/common_cpp/ipc/include/shared_types.hpp b/data_acq_and_processing/common_cpp/ipc/include/shared_types.hpp
index 93a29c9..de6548e 100644
--- a/data_acq_and_processing/common_cpp/ipc/include/shared_types.hpp
+++ b/data_acq_and_processing/common_cpp/ipc/include/shared_types.hpp
@@ -38,6 +38,13 @@ struct SweepTraceBlock {
     std::vector<Complex32> s11{};
     // Complex S21 samples for matching frequency points.
     std::vector<Complex32> s21{};
+    // Monotonic window in which THIS trace's sweep was measured, excluding the
+    // switch drive and settling that preceded it. In a switched matrix the
+    // collection is assembled combo by combo over many milliseconds, so the
+    // collection-level window says nothing about when an individual combo was
+    // measured. Zero on both is a valid "unmeasured" sentinel.
+    std::uint64_t capture_start_ns = 0;
+    std::uint64_t capture_end_ns = 0;
 };
 
 struct RawSweepCollection {
diff --git a/data_acq_and_processing/common_cpp/ipc/src/shared_types.cpp b/data_acq_and_processing/common_cpp/ipc/src/shared_types.cpp
index fa45072..244e7ac 100644
--- a/data_acq_and_processing/common_cpp/ipc/src/shared_types.cpp
+++ b/data_acq_and_processing/common_cpp/ipc/src/shared_types.cpp
@@ -172,6 +172,10 @@ void require_count_fits(std::uint32_t count, std::size_t min_bytes_each, BinaryR
     return trace;
 }
 
+// The capture windows live in a TRAILER after the trace blocks rather than inside
+// them, so a reader built before they existed still decodes every trace and simply
+// stops early. The trailer grows the same way: collection window first, then the
+// per-trace window table (one pair per trace, in trace order).
 void write_trace_collection(BinaryWriter& writer, std::uint32_t magic, const RawSweepCollection& collection) {
     writer.write(magic);
     writer.write(collection.collection_id);
@@ -184,6 +188,12 @@ void write_trace_collection(BinaryWriter& writer, std::uint32_t magic, const Raw
 
     writer.write(collection.capture_start_ns);
     writer.write(collection.capture_end_ns);
+
+    writer.write(checked_count_to_u32(collection.traces.size(), "Trace capture window count"));
+    for (const auto& trace : collection.traces) {
+        writer.write(trace.capture_start_ns);
+        writer.write(trace.capture_end_ns);
+    }
 }
 
 [[nodiscard]] auto read_trace_collection(BinaryReader& reader, std::uint32_t expected_magic) -> RawSweepCollection {
@@ -204,16 +214,31 @@ void write_trace_collection(BinaryWriter& writer, std::uint32_t magic, const Raw
         collection.traces.push_back(read_trace_block(reader));
     }
 
+    // Each trailer stage is optional: a payload from an older producer stops after
+    // the trace blocks (or after the collection window) and leaves the rest zeroed.
     if (reader.remaining_bytes() == 0U) {
         return collection;
     }
-    if (reader.remaining_bytes() != (sizeof(std::uint64_t) * 2U)) {
-        throw std::runtime_error("Unexpected trailing bytes in trace collection");
+    if (reader.remaining_bytes() < (sizeof(std::uint64_t) * 2U)) {
+        throw std::runtime_error("Truncated capture window in trace collection");
     }
 
     collection.capture_start_ns = reader.read<std::uint64_t>();
     collection.capture_end_ns = reader.read<std::uint64_t>();
 
+    if (reader.remaining_bytes() == 0U) {
+        return collection;
+    }
+
+    const auto trace_time_count = reader.read<std::uint32_t>();
+    if (trace_time_count != collection.traces.size()) {
+        throw std::runtime_error("Per-trace capture window count does not match trace count");
+    }
+    for (auto& trace : collection.traces) {
+        trace.capture_start_ns = reader.read<std::uint64_t>();
+        trace.capture_end_ns = reader.read<std::uint64_t>();
+    }
+
     return collection;
 }
 
diff --git a/data_acq_and_processing/preprocessing/calibration_master/src/calibration_master.cpp b/data_acq_and_processing/preprocessing/calibration_master/src/calibration_master.cpp
index f9ae484..0e5e126 100644
--- a/data_acq_and_processing/preprocessing/calibration_master/src/calibration_master.cpp
+++ b/data_acq_and_processing/preprocessing/calibration_master/src/calibration_master.cpp
@@ -34,6 +34,9 @@ auto CalibrationMaster::apply_to_trace(const ipc::SweepTraceBlock& measured_trac
     output.frequency_hz = measured_trace.frequency_hz;
     output.s21 = apply_s21(measured_trace.combo, measured_trace.frequency_hz, measured_trace.s21);
     output.s11 = apply_s11(measured_trace.combo, measured_trace.frequency_hz, measured_trace.s11);
+    // Calibration reshapes the samples, not when they were measured.
+    output.capture_start_ns = measured_trace.capture_start_ns;
+    output.capture_end_ns = measured_trace.capture_end_ns;
     return output;
 }
 
diff --git a/data_acq_and_processing/preprocessing/reference_master/src/reference_master.cpp b/data_acq_and_processing/preprocessing/reference_master/src/reference_master.cpp
index afea007..accad52 100644
--- a/data_acq_and_processing/preprocessing/reference_master/src/reference_master.cpp
+++ b/data_acq_and_processing/preprocessing/reference_master/src/reference_master.cpp
@@ -132,6 +132,9 @@ auto ReferenceMaster::apply_to_trace(const ipc::SweepTraceBlock& calibrated_trac
     output.frequency_hz = calibrated_trace.frequency_hz;
     output.s21 = apply_s21(calibrated_trace.combo, calibrated_trace.frequency_hz, calibrated_trace.s21);
     output.s11 = apply_s11(calibrated_trace.combo, calibrated_trace.frequency_hz, calibrated_trace.s11);
+    // Reference subtraction reshapes the samples, not when they were measured.
+    output.capture_start_ns = calibrated_trace.capture_start_ns;
+    output.capture_end_ns = calibrated_trace.capture_end_ns;
     return output;
 }
 
diff --git a/data_acq_and_processing/sweep_orchestrator/src/sweep_orchestrator.cpp b/data_acq_and_processing/sweep_orchestrator/src/sweep_orchestrator.cpp
index 8a44b3c..76661ba 100644
--- a/data_acq_and_processing/sweep_orchestrator/src/sweep_orchestrator.cpp
+++ b/data_acq_and_processing/sweep_orchestrator/src/sweep_orchestrator.cpp
@@ -147,14 +147,18 @@ class DriverLifecycleGuard {
 // Worst-case serialized size of a collection given the configured combo count and sweep
 // point count, using the trace wire format (see ipc::write_trace_collection/write_trace_block):
 //   collection header: magic(4) + collection_id(8) + monotonic_ns(8) + trace_count(4)
-//                     + capture_start_ns(8) + capture_end_ns(8) = 40 bytes
+//                     + capture_start_ns(8) + capture_end_ns(8)
+//                     + trace capture window count(4) = 44 bytes
 //   per trace block:   input_pos(4) + output_pos(4) + point_count(4) = 12 bytes
 //                     + per point: frequency(4) + s11(8) + s21(8) = 20 bytes
+//                     + trailer: capture_start_ns(8) + capture_end_ns(8) = 16 bytes
 [[nodiscard]] auto worst_case_serialized_bytes(std::size_t combo_count, std::uint32_t sweep_points) -> std::size_t {
-    constexpr std::size_t kCollectionHeaderBytes = 40U;
+    constexpr std::size_t kCollectionHeaderBytes = 44U;
     constexpr std::size_t kTraceHeaderBytes = 12U;
     constexpr std::size_t kBytesPerPoint = 20U;
-    const std::size_t per_trace = kTraceHeaderBytes + (static_cast<std::size_t>(sweep_points) * kBytesPerPoint);
+    constexpr std::size_t kTraceTrailerBytes = 16U;
+    const std::size_t per_trace =
+        kTraceHeaderBytes + (static_cast<std::size_t>(sweep_points) * kBytesPerPoint) + kTraceTrailerBytes;
     return kCollectionHeaderBytes + (combo_count * per_trace);
 }
 
@@ -290,7 +294,12 @@ auto SweepOrchestrator::acquire_one_collection(
         // Production drivers ignore this; mock drivers use it to give every
         // (input, output) pair its own synthetic response.
         radar_driver_.set_active_combo(combo);
+        // Stamp around the sweep only: the switch drive and settling above belong to
+        // neither the previous combo nor this one, so excluding them keeps the window
+        // an honest "when was this combo actually measured".
+        const auto sweep_start_ns = ipc::current_monotonic_ns();
         auto sweep = radar_driver_.acquire_sweep();
+        const auto sweep_end_ns = ipc::current_monotonic_ns();
         validate_sweep(sweep);
 
         ipc::SweepTraceBlock trace{};
@@ -298,6 +307,8 @@ auto SweepOrchestrator::acquire_one_collection(
         trace.frequency_hz = std::move(sweep.frequency_hz);
         trace.s11 = std::move(sweep.s11);
         trace.s21 = std::move(sweep.s21);
+        trace.capture_start_ns = sweep_start_ns;
+        trace.capture_end_ns = sweep_end_ns;
         collection.traces.push_back(std::move(trace));
     }
 
diff --git a/python_app/hardware_full/multi_device_service.py b/python_app/hardware_full/multi_device_service.py
index 7d10ba7..8d80dca 100644
--- a/python_app/hardware_full/multi_device_service.py
+++ b/python_app/hardware_full/multi_device_service.py
@@ -332,10 +332,15 @@ class MultiDeviceLibreVnaService:
         assert self._sweep_configuration is not None
 
         self._controller.configure_continuous_sweep(self._sweep_configuration)
+        # Bound the sweep itself rather than reusing `capture_start_ns`: the latter
+        # is taken before any retry/recovery, so it would overstate how long the
+        # traces below took to measure.
+        sweep_start_ns = time.monotonic_ns()
         result = self._controller.collect_running_sweep_cycles(
             1,
             datapoint_timeout_seconds=LIBREVNA_NATIVE_SWEEP_TIMEOUT_SECONDS,
         )
+        sweep_end_ns = time.monotonic_ns()
         normalized_s_parameters = {
             str(name).lower(): np.asarray(values, dtype=np.complex64)
             for name, values in result.s_parameters.items()
@@ -356,6 +361,11 @@ class MultiDeviceLibreVnaService:
                         frequency_hz=frequencies,
                         s11=reflection,
                         s21=self._required_s_parameter(normalized_s_parameters, s_parameter_name),
+                        # Every combo comes out of the same synchronized cycle, so
+                        # they all share one window — no combo was measured earlier
+                        # or later than another here.
+                        capture_start_ns=sweep_start_ns,
+                        capture_end_ns=sweep_end_ns,
                     )
                 )
 
@@ -368,6 +378,7 @@ class MultiDeviceLibreVnaService:
 
     def _acquire_mock_collection(self, collection_id: int, capture_start_ns: int) -> SweepCollection:
         assert self._sweep_configuration is not None
+        mock_sweep_start_ns = time.monotonic_ns()
         points = int(self._sweep_configuration.points)
         frequencies = np.linspace(
             self._sweep_configuration.start_hz,
@@ -393,6 +404,8 @@ class MultiDeviceLibreVnaService:
                         frequency_hz=frequencies,
                         s11=s11,
                         s21=s21,
+                        capture_start_ns=mock_sweep_start_ns,
+                        capture_end_ns=time.monotonic_ns(),
                     )
                 )
         self._mock_phase += 0.05
diff --git a/python_app/hardware_full/sn9000_service.py b/python_app/hardware_full/sn9000_service.py
index d14e612..2da22b3 100644
--- a/python_app/hardware_full/sn9000_service.py
+++ b/python_app/hardware_full/sn9000_service.py
@@ -183,7 +183,11 @@ class Sn9000Service:
         capture_start_ns = time.monotonic_ns()
 
         s_parameters = self._query_sweep_s_parameters(points)
-        traces = self._assemble_traces(s_parameters)
+        traces = self._assemble_traces(
+            s_parameters,
+            sweep_start_ns=capture_start_ns,
+            sweep_end_ns=time.monotonic_ns(),
+        )
 
         return SweepCollection(
             collection_id=int(collection_id),
@@ -256,7 +260,13 @@ class Sn9000Service:
     def _uses_pyvisa_py_backend(self) -> bool:
         return self.visa_library == "@py" or self.visa_library.endswith("@py")
 
-    def _assemble_traces(self, s_parameters: dict[str, np.ndarray]) -> list[TraceData]:
+    def _assemble_traces(
+        self,
+        s_parameters: dict[str, np.ndarray],
+        *,
+        sweep_start_ns: int,
+        sweep_end_ns: int,
+    ) -> list[TraceData]:
         frequency_hz = self._require_frequency_axis()
         traces: list[TraceData] = []
         for output_position, output_port in enumerate(_OUTPUT_PORT_BY_INDEX):
@@ -269,6 +279,10 @@ class Sn9000Service:
                         frequency_hz=frequency_hz,
                         s11=reflection,
                         s21=transmission,
+                        # One triggered sweep produces every port pair at once, so
+                        # all combos share the sweep's window.
+                        capture_start_ns=int(sweep_start_ns),
+                        capture_end_ns=int(sweep_end_ns),
                     )
                 )
         return traces
diff --git a/python_app/hardware_full/switched_matrix_radar_service.py b/python_app/hardware_full/switched_matrix_radar_service.py
index 5b4437c..22ff2ad 100644
--- a/python_app/hardware_full/switched_matrix_radar_service.py
+++ b/python_app/hardware_full/switched_matrix_radar_service.py
@@ -140,7 +140,13 @@ class SwitchedMatrixRadarService:
         )
 
     def _acquire_step_traces(self, out_k: int, in_k: int, collection_id: int) -> list[TraceData]:
-        """Drive both switches to one step, settle, and collect its widened traces."""
+        """Drive both switches to one step, settle, and collect its widened traces.
+
+        Every returned trace carries the monotonic window of the inner collection
+        that produced it, so a consumer can tell when each combo of a switched
+        matrix was really measured instead of only when the whole cycle began and
+        ended. The switch drive and settling are deliberately outside the window.
+        """
         step_start_ns = time.monotonic_ns()
         if self.output_switch is not None:
             self.output_switch.switch_to(out_k)
@@ -178,6 +184,11 @@ class SwitchedMatrixRadarService:
                     input=in_k * self.inner_input_positions + int(trace.combo.input),
                     output=out_k * self.inner_output_positions + int(trace.combo.output),
                 ),
+                # Keep the inner service's own per-trace window when it reports one
+                # (it knows its internal port order better than this step does);
+                # otherwise fall back to the window of this inner collection.
+                capture_start_ns=int(trace.capture_start_ns) or settled_ns,
+                capture_end_ns=int(trace.capture_end_ns) or inner_end_ns,
             )
             for trace in sub.traces
         ]
diff --git a/python_app/models/dataset_model.py b/python_app/models/dataset_model.py
index 9154144..b3a593e 100644
--- a/python_app/models/dataset_model.py
+++ b/python_app/models/dataset_model.py
@@ -32,12 +32,22 @@ class ComboKey:
 
 @dataclass(slots=True)
 class TraceData:
-    """One frequency-domain trace set for a specific switch combination."""
+    """One frequency-domain trace set for a specific switch combination.
+
+    ``capture_start_ns``/``capture_end_ns`` bound the monotonic window in which
+    THIS trace's sweep was measured, excluding the switch drive and settling that
+    preceded it. In switched modes a collection is assembled combo by combo over
+    many milliseconds, so the collection-level window says nothing about when any
+    individual combo was measured — these do. Zero on both means the producer did
+    not report per-trace timing.
+    """
 
     combo: ComboKey
     frequency_hz: np.ndarray
     s11: np.ndarray
     s21: np.ndarray
+    capture_start_ns: int = 0
+    capture_end_ns: int = 0
 
 
 @dataclass(slots=True)
diff --git a/python_app/orchestration/shm/decoder.py b/python_app/orchestration/shm/decoder.py
index cfc3e6b..5ab3622 100644
--- a/python_app/orchestration/shm/decoder.py
+++ b/python_app/orchestration/shm/decoder.py
@@ -54,12 +54,27 @@ def decode_trace_collection(payload: bytes, expected_magic: int) -> SweepCollect
             )
         )
 
+    # Optional trailer, written after the trace blocks by newer producers: the
+    # collection capture window, then a per-trace window table. Both stages are
+    # optional so payloads from an older producer still decode (the timestamps
+    # simply stay zero).
     capture_start_ns = 0
     capture_end_ns = 0
-    if cursor.remaining_bytes() == 16:
+    if cursor.remaining_bytes() != 0:
+        if cursor.remaining_bytes() < 16:
+            raise ValueError("Truncated capture window in trace collection")
         capture_start_ns = cursor.read_u64()
         capture_end_ns = cursor.read_u64()
-    elif cursor.remaining_bytes() != 0:
+
+    if cursor.remaining_bytes() != 0:
+        trace_time_count = cursor.read_u32()
+        if trace_time_count != len(traces):
+            raise ValueError("Per-trace capture window count does not match trace count")
+        for trace in traces:
+            trace.capture_start_ns = cursor.read_u64()
+            trace.capture_end_ns = cursor.read_u64()
+
+    if cursor.remaining_bytes() != 0:
         raise ValueError("Unexpected trailing bytes in trace collection")
 
     return SweepCollection(
diff --git a/python_app/scripts/convert_snapshot_to_vna_history.py b/python_app/scripts/convert_snapshot_to_vna_history.py
index 21449a9..828d992 100644
--- a/python_app/scripts/convert_snapshot_to_vna_history.py
+++ b/python_app/scripts/convert_snapshot_to_vna_history.py
@@ -23,6 +23,9 @@ class TraceRecord:
     stage_index: int
     frequency_hz: np.ndarray
     samples: np.ndarray
+    # End of this trace's own sweep, from the snapshot's per-trace metadata; 0 for a
+    # snapshot recorded before per-trace timing existed.
+    capture_end_ns: int = 0
 
 
 @dataclass(frozen=True)
@@ -128,6 +131,7 @@ def _load_stage_records(
                 stage_index=_parse_stage_index(collection_dir.name, fallback_idx),
                 frequency_hz=frequency_hz,
                 samples=samples,
+                capture_end_ns=int(trace_meta.get("capture_end_ns", 0)),
             )
         )
 
@@ -214,7 +218,15 @@ def _build_sweep_history(
 
         start_freq_hz = float(base.frequency_hz[0])
         stop_freq_hz = float(base.frequency_hz[-1])
-        timestamp_sec = float(base.monotonic_ns) / 1_000_000_000.0 if base.monotonic_ns > 0 else float(fallback_index)
+        # Prefer this trace's own sweep time: with a switching matrix the combos of
+        # one collection are measured milliseconds apart, so the collection
+        # timestamp misplaces every combo but the last.
+        if base.capture_end_ns > 0:
+            timestamp_sec = float(base.capture_end_ns) / 1_000_000_000.0
+        elif base.monotonic_ns > 0:
+            timestamp_sec = float(base.monotonic_ns) / 1_000_000_000.0
+        else:
+            timestamp_sec = float(fallback_index)
 
         history.append(
             {
diff --git a/python_app/scripts/kamil_adc_raw_producer.py b/python_app/scripts/kamil_adc_raw_producer.py
index c4280f9..b3b25dd 100644
--- a/python_app/scripts/kamil_adc_raw_producer.py
+++ b/python_app/scripts/kamil_adc_raw_producer.py
@@ -184,12 +184,16 @@ def main() -> int:
                     if collector_driven:
                         # The collector already switched and tagged the sweep; just
                         # read the clean capture for this combination.
+                        sweep_start_ns = time.monotonic_ns()
                         sweep = radar.acquire(combo=(combo.input, combo.output))
                     else:
                         output_switch.switch_to(combo.output)
                         input_switch.switch_to(combo.input)
                         if config.runtime.settling_ms > 0:
                             time.sleep(config.runtime.settling_ms / 1000.0)
+                        # Stamped after switching and settling so the window covers
+                        # the sweep alone, not the dead time before it.
+                        sweep_start_ns = time.monotonic_ns()
                         sweep = radar.acquire()
                     traces.append(
                         TraceData(
@@ -197,6 +201,8 @@ def main() -> int:
                             frequency_hz=np.asarray(sweep.x, dtype=np.float32),
                             s11=np.asarray(sweep.trace("s11"), dtype=np.complex64),
                             s21=np.asarray(sweep.trace("s21"), dtype=np.complex64),
+                            capture_start_ns=sweep_start_ns,
+                            capture_end_ns=time.monotonic_ns(),
                         )
                     )
             except Exception as exc:  # noqa: BLE001 — reconnect forever, never give up
diff --git a/python_app/storage/npz/serialize.py b/python_app/storage/npz/serialize.py
index 65848af..8f727b0 100644
--- a/python_app/storage/npz/serialize.py
+++ b/python_app/storage/npz/serialize.py
@@ -22,7 +22,14 @@ def _write_interleaved_complex(buffer: bytearray, values: np.ndarray) -> None:
 
 
 def serialize_trace_collection(collection: SweepCollection, magic: int) -> bytes:
-    """Serialize one raw/preprocessed trace collection into ring-compatible binary format."""
+    """Serialize one raw/preprocessed trace collection into ring-compatible binary format.
+
+    The trailer is appended after the trace blocks so older readers, which stop at
+    the last block, still decode the traces: first the collection capture window,
+    then a per-trace window table (one ``(start_ns, end_ns)`` pair per trace, in
+    trace order). See :func:`python_app.orchestration.shm.decoder.decode_trace_collection`
+    and ``read_trace_collection`` in ``common_cpp/ipc/src/shared_types.cpp``.
+    """
     buffer = bytearray()
     buffer.extend(struct.pack("<IQQI", magic, collection.collection_id, collection.monotonic_ns, len(collection.traces)))
 
@@ -48,6 +55,11 @@ def serialize_trace_collection(collection: SweepCollection, magic: int) -> bytes
             int(collection.capture_end_ns),
         )
     )
+    buffer.extend(struct.pack("<I", len(collection.traces)))
+    for trace in collection.traces:
+        buffer.extend(
+            struct.pack("<QQ", int(trace.capture_start_ns), int(trace.capture_end_ns))
+        )
     return bytes(buffer)
 
 
diff --git a/python_app/storage/npz/snapshot_numpy.py b/python_app/storage/npz/snapshot_numpy.py
index 29df9c3..dff5df6 100644
--- a/python_app/storage/npz/snapshot_numpy.py
+++ b/python_app/storage/npz/snapshot_numpy.py
@@ -124,6 +124,17 @@ def save_trace_history_binary(stage_dir: Path, history: list[SweepCollection], m
                     "capture_start_ns": int(collection.capture_start_ns),
                     "capture_end_ns": int(collection.capture_end_ns),
                     "trace_count": len(collection.traces),
+                    # Also in the .bin trailer; repeated here so per-combo timing is
+                    # readable without decoding the binary payload.
+                    "traces": [
+                        {
+                            "input": int(trace.combo.input),
+                            "output": int(trace.combo.output),
+                            "capture_start_ns": int(trace.capture_start_ns),
+                            "capture_end_ns": int(trace.capture_end_ns),
+                        }
+                        for trace in collection.traces
+                    ],
                 },
                 indent=2,
             ),
@@ -182,6 +193,10 @@ def save_trace_history_numpy(
                     "input": int(trace.combo.input),
                     "output": int(trace.combo.output),
                     "points": int(freq.size),
+                    # When each combo was measured, which in a switched matrix is
+                    # spread across the collection window rather than aligned with it.
+                    "capture_start_ns": int(trace.capture_start_ns),
+                    "capture_end_ns": int(trace.capture_end_ns),
                     "freq_file": f"{tag}_freq.npy",
                     "s11_file": f"{tag}_s11.npy",
                     "s21_file": f"{tag}_s21.npy",
diff --git a/python_app/storage/npz/store.py b/python_app/storage/npz/store.py
index dfdd26a..17d5a12 100644
--- a/python_app/storage/npz/store.py
+++ b/python_app/storage/npz/store.py
@@ -117,6 +117,8 @@ class NpzStore(StoreApi):
                 {
                     "input": trace.combo.input,
                     "output": trace.combo.output,
+                    "capture_start_ns": int(trace.capture_start_ns),
+                    "capture_end_ns": int(trace.capture_end_ns),
                     "freq_key": freq_key,
                     "s11_key": s11_key,
                     "s21_key": s21_key,
@@ -177,6 +179,8 @@ class NpzStore(StoreApi):
                         frequency_hz=freq,
                         s11=s11,
                         s21=s21,
+                        capture_start_ns=int(combo.get("capture_start_ns", 0)),
+                        capture_end_ns=int(combo.get("capture_end_ns", 0)),
                     )
                 )
 
diff --git a/python_app/storage/npz/vna_history_json.py b/python_app/storage/npz/vna_history_json.py
index 955448c..37884e5 100644
--- a/python_app/storage/npz/vna_history_json.py
+++ b/python_app/storage/npz/vna_history_json.py
@@ -25,6 +25,10 @@ class TraceRecord:
     stage_index: int
     frequency_hz: np.ndarray
     samples: np.ndarray
+    # End of this trace's own sweep, or 0 when the producer reported no per-trace
+    # timing. Preferred over the collection timestamp for the exported sweep time:
+    # in a switched matrix each combo is measured at a different instant.
+    capture_end_ns: int = 0
 
 
 def _normalize_channel(channel: str) -> str:
@@ -85,6 +89,7 @@ def _build_stage_records(
                 stage_index=int(stage_index),
                 frequency_hz=frequency_hz,
                 samples=samples,
+                capture_end_ns=int(trace.capture_end_ns),
             )
         )
     return records
@@ -137,7 +142,15 @@ def _build_sweep_history(
 
         start_freq_hz = float(base.frequency_hz[0])
         stop_freq_hz = float(base.frequency_hz[-1])
-        timestamp_sec = float(base.monotonic_ns) / 1_000_000_000.0 if base.monotonic_ns > 0 else float(fallback_index)
+        # Prefer the exported trace's own sweep time: with a switching matrix the
+        # combos of one collection are measured milliseconds apart, so the
+        # collection timestamp misplaces every combo but the last.
+        if base.capture_end_ns > 0:
+            timestamp_sec = float(base.capture_end_ns) / 1_000_000_000.0
+        elif base.monotonic_ns > 0:
+            timestamp_sec = float(base.monotonic_ns) / 1_000_000_000.0
+        else:
+            timestamp_sec = float(fallback_index)
 
         history.append(
             {
diff --git a/python_app/tests/test_shm_ipc.py b/python_app/tests/test_shm_ipc.py
index 10f92f2..89f4a31 100644
--- a/python_app/tests/test_shm_ipc.py
+++ b/python_app/tests/test_shm_ipc.py
@@ -39,12 +39,19 @@ from python_app.orchestration.shm.ring_writer import ShmRingWriter
 from python_app.storage.npz.serialize import serialize_result_collection, serialize_trace_collection
 
 
-def _trace(in_pos: int, out_pos: int, n: int) -> TraceData:
+def _trace(in_pos: int, out_pos: int, n: int, *, capture_ns: tuple[int, int] = (0, 0)) -> TraceData:
     """Build a trace with float32-exact data so round-trips compare exactly."""
     freq = np.arange(n, dtype=np.float32) + 1.0
     s11 = (np.arange(n, dtype=np.float32) + 0.5j * np.arange(n, dtype=np.float32)).astype(np.complex64)
     s21 = (-np.arange(n, dtype=np.float32) + 2.0j * np.arange(n, dtype=np.float32)).astype(np.complex64)
-    return TraceData(combo=ComboKey(input=in_pos, output=out_pos), frequency_hz=freq, s11=s11, s21=s21)
+    return TraceData(
+        combo=ComboKey(input=in_pos, output=out_pos),
+        frequency_hz=freq,
+        s11=s11,
+        s21=s21,
+        capture_start_ns=capture_ns[0],
+        capture_end_ns=capture_ns[1],
+    )
 
 
 class TraceCollectionRoundTripTest(unittest.TestCase):
@@ -52,7 +59,7 @@ class TraceCollectionRoundTripTest(unittest.TestCase):
         collection = SweepCollection(
             collection_id=7,
             monotonic_ns=123,
-            traces=[_trace(0, 0, 4), _trace(3, 1, 2)],
+            traces=[_trace(0, 0, 4, capture_ns=(11, 13)), _trace(3, 1, 2, capture_ns=(15, 19))],
             capture_start_ns=10,
             capture_end_ns=20,
         )
@@ -66,6 +73,10 @@ class TraceCollectionRoundTripTest(unittest.TestCase):
             self.assertTrue(np.array_equal(got.frequency_hz, original.frequency_hz))
             self.assertTrue(np.array_equal(got.s11, original.s11))
             self.assertTrue(np.array_equal(got.s21, original.s21))
+            self.assertEqual(
+                (got.capture_start_ns, got.capture_end_ns),
+                (original.capture_start_ns, original.capture_end_ns),
+            )
 
     def test_raw_round_trips(self) -> None:
         self._assert_round_trips(RAW_MAGIC)
@@ -78,6 +89,34 @@ class TraceCollectionRoundTripTest(unittest.TestCase):
         decoded = decode_trace_collection(serialize_trace_collection(collection, RAW_MAGIC), RAW_MAGIC)
         self.assertEqual(decoded.traces, [])
 
+    def test_payload_without_per_trace_window_table_still_decodes(self) -> None:
+        # A producer built before per-trace timing stops after the collection
+        # window; its traces must still decode, with the timestamps left at zero.
+        collection = SweepCollection(
+            collection_id=4,
+            monotonic_ns=5,
+            traces=[_trace(1, 0, 3, capture_ns=(7, 9))],
+            capture_start_ns=6,
+            capture_end_ns=10,
+        )
+        full = serialize_trace_collection(collection, RAW_MAGIC)
+        legacy = full[: -(4 + 16 * len(collection.traces))]
+
+        decoded = decode_trace_collection(legacy, RAW_MAGIC)
+        self.assertEqual((decoded.capture_start_ns, decoded.capture_end_ns), (6, 10))
+        self.assertEqual(len(decoded.traces), 1)
+        self.assertEqual((decoded.traces[0].capture_start_ns, decoded.traces[0].capture_end_ns), (0, 0))
+
+    def test_per_trace_window_count_mismatch_is_rejected(self) -> None:
+        collection = SweepCollection(
+            collection_id=4, monotonic_ns=5, traces=[_trace(1, 0, 3, capture_ns=(7, 9))]
+        )
+        payload = serialize_trace_collection(collection, RAW_MAGIC)
+        # Overwrite the window-table count (u32 before the single 16-byte pair).
+        corrupt = payload[:-20] + struct.pack("<I", 2) + payload[-16:]
+        with self.assertRaises(ValueError):
+            decode_trace_collection(corrupt, RAW_MAGIC)
+
 
 class ResultCollectionRoundTripTest(unittest.TestCase):
     def test_all_payload_kinds_round_trip(self) -> None:
diff --git a/python_app/tests/test_switched_matrix_capture.py b/python_app/tests/test_switched_matrix_capture.py
index 49c030f..71fbf38 100644
--- a/python_app/tests/test_switched_matrix_capture.py
+++ b/python_app/tests/test_switched_matrix_capture.py
@@ -140,6 +140,36 @@ class SwitchedMatrixComboAcquisitionTest(unittest.TestCase):
             expected_combos,
         )
 
+    def test_each_switch_step_stamps_its_traces_with_its_own_capture_window(self) -> None:
+        # The whole point of per-trace timing: three switch steps are measured one
+        # after another, so their traces must NOT all share the collection window.
+        service, _inner, _input_switch = _switched_service(input_steps=3)
+
+        collection = service.acquire_collection(collection_id=7)
+
+        windows_by_step: dict[int, set[tuple[int, int]]] = {}
+        for trace in collection.traces:
+            step = int(trace.combo.input) // _INNER_INPUTS
+            windows_by_step.setdefault(step, set()).add(
+                (int(trace.capture_start_ns), int(trace.capture_end_ns))
+            )
+
+        self.assertEqual(sorted(windows_by_step), [0, 1, 2])
+        for step, windows in windows_by_step.items():
+            self.assertEqual(len(windows), 1, f"step {step} traces disagree on their window")
+            start_ns, end_ns = next(iter(windows))
+            self.assertGreater(start_ns, 0)
+            self.assertGreaterEqual(end_ns, start_ns)
+            # Each step's window sits inside the collection's.
+            self.assertGreaterEqual(start_ns, collection.capture_start_ns)
+            self.assertLessEqual(end_ns, collection.capture_end_ns)
+
+        # Steps are strictly ordered in time — the whole reason the collection-level
+        # window cannot stand in for a per-combo timestamp.
+        step_starts = [next(iter(windows_by_step[step]))[0] for step in sorted(windows_by_step)]
+        self.assertEqual(step_starts, sorted(step_starts))
+        self.assertGreater(len(set(step_starts)), 1)
+
 
 class ManualComboCaptureUsesTargetedAcquisitionTest(unittest.TestCase):
     """The per-combo capture session must not sweep the full widened matrix."""
diff --git a/python_app/workflows/multi_radar_capture_workflow.py b/python_app/workflows/multi_radar_capture_workflow.py
index 3a9cec5..db03007 100644
--- a/python_app/workflows/multi_radar_capture_workflow.py
+++ b/python_app/workflows/multi_radar_capture_workflow.py
@@ -232,6 +232,7 @@ class MultiRadarSequentialCaptureSession:
                     time.sleep(self._base_config.runtime.settling_ms / 1000.0)
                 sweep_traces: list[TraceData] = []
                 for _ in range(self._median_sweep_count):
+                    sweep_start_ns = time.monotonic_ns()
                     sweep = self._radar.acquire()
                     sweep_traces.append(
                         TraceData(
@@ -239,6 +240,8 @@ class MultiRadarSequentialCaptureSession:
                             frequency_hz=np.asarray(sweep.x, dtype=np.float32),
                             s11=np.asarray(sweep.trace("s11"), dtype=np.complex64),
                             s21=np.asarray(sweep.trace("s21"), dtype=np.complex64),
+                            capture_start_ns=sweep_start_ns,
+                            capture_end_ns=time.monotonic_ns(),
                         )
                     )
                 trace = combine_traces_via_median(sweep_traces)
diff --git a/python_app/workflows/sequential_capture_workflow.py b/python_app/workflows/sequential_capture_workflow.py
index ad3f53a..bdd67f2 100644
--- a/python_app/workflows/sequential_capture_workflow.py
+++ b/python_app/workflows/sequential_capture_workflow.py
@@ -202,6 +202,7 @@ class SequentialCaptureSession:
 
         sweep_traces: list[TraceData] = []
         for _ in range(self._median_sweep_count):
+            sweep_start_ns = time.monotonic_ns()
             sweep = self._radar.acquire()
             sweep_traces.append(
                 TraceData(
@@ -209,6 +210,8 @@ class SequentialCaptureSession:
                     frequency_hz=np.asarray(sweep.x, dtype=np.float32),
                     s11=np.asarray(sweep.trace("s11"), dtype=np.complex64),
                     s21=np.asarray(sweep.trace("s21"), dtype=np.complex64),
+                    capture_start_ns=sweep_start_ns,
+                    capture_end_ns=time.monotonic_ns(),
                 )
             )
         trace = combine_traces_via_median(sweep_traces)
@@ -385,11 +388,16 @@ def combine_traces_via_median(traces: list[TraceData]) -> TraceData:
     s21_median = (
         np.median(s21_stack.real, axis=0) + 1j * np.median(s21_stack.imag, axis=0)
     ).astype(np.complex64)
+    # The median is built from every input sweep, so its window spans all of them.
+    capture_starts = [int(t.capture_start_ns) for t in traces if int(t.capture_start_ns) > 0]
+    capture_ends = [int(t.capture_end_ns) for t in traces if int(t.capture_end_ns) > 0]
     return TraceData(
         combo=ComboKey(input=int(combo.input), output=int(combo.output)),
         frequency_hz=np.asarray(first.frequency_hz, dtype=np.float32),
         s11=s11_median,
         s21=s21_median,
+        capture_start_ns=min(capture_starts) if capture_starts else 0,
+        capture_end_ns=max(capture_ends) if capture_ends else 0,
     )