diff --git a/README.md b/README.md
index 8736644..7039656 100644
--- a/README.md
+++ b/README.md
@@ -155,6 +155,7 @@ Cooldown behavior:
 - [`docs/reviewer-brief.md`](docs/reviewer-brief.md) gives the short problem, value, evidence, and boundary summary
 - [`docs/reviewer-path.md`](docs/reviewer-path.md) maps common review questions to the right demo and artifacts
 - [`docs/architecture.md`](docs/architecture.md) diagrams the local file-based detection workflow
+- [`docs/event-time-model.md`](docs/event-time-model.md) defines event, observed, window, and artifact time semantics
 - [`docs/sample-output.md`](docs/sample-output.md) summarizes the committed sample artifacts
 - [`docs/roadmap.md`](docs/roadmap.md) sketches the v0.7 / v1.0 consolidation direction
 - [`data/processed/summary.json`](data/processed/summary.json) captures the default run in machine-readable form
diff --git a/docs/README.md b/docs/README.md
index 8b5c969..4ea4175 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -13,6 +13,7 @@ This directory separates the current reviewer route from supporting design notes
 ## Supporting docs
 
 - [`sample-output.md`](sample-output.md): committed output counts and sample artifacts
+- [`event-time-model.md`](event-time-model.md): event, observed, window, and artifact time semantics
 - [`design-notes.md`](design-notes.md): original telemetry-window design boundaries
 - [`ai-assisted-detection-design.md`](ai-assisted-detection-design.md): bounded AI-assisted detection design
 - [`ai-assisted-detection-examples.md`](ai-assisted-detection-examples.md): example AI-assisted detection outputs and guardrail behavior
diff --git a/docs/event-time-model.md b/docs/event-time-model.md
new file mode 100644
index 0000000..60b8797
--- /dev/null
+++ b/docs/event-time-model.md
@@ -0,0 +1,38 @@
+# Event Time Model
+
+`telemetry-lab` keeps event, observation, window, and artifact times separate so local detection demos remain reproducible and auditable.
+
+This model is informed by the OpenTelemetry Logs Data Model distinction between `Timestamp` and `ObservedTimestamp`: `Timestamp` represents when the event occurred at the source, while `ObservedTimestamp` represents when a collection system observed it. OpenTelemetry recommends using `Timestamp` first when exporting to a format with only one timestamp, falling back to `ObservedTimestamp` only when `Timestamp` is missing.
+
+`telemetry-lab` is not an OpenTelemetry implementation. The terms below define how this repository names and interprets time in sample inputs, generated features, alerts, reports, and future demo artifacts.
+
+## Fields
+
+| Field | Meaning | Used for detection ordering? | Current repository mapping |
+| --- | --- | --- | --- |
+| `event_time` | Time the source event happened. | Yes | The default input column is named `timestamp`; configs may use `time.timestamp_col` to point at a source column such as `event_time`. |
+| `observed_time` | Time a collector, loader, or intermediary observed the event. | No, unless a demo explicitly documents fallback behavior. | Optional future input or artifact field. Current core demos do not require it. |
+| `window_start` / `window_end` | Deterministic analysis interval derived from `event_time`. | Yes | Feature rows, alert rows, and dedup artifacts use these boundaries. Windows are treated as `[window_start, window_end)`. |
+| `artifact_generated_at` | Time an output artifact was rendered or written. | No | Optional provenance metadata for reports, summaries, or reviewer packs. It must not be used as event evidence. |
+
+## Rules
+
+- Prefer `event_time` for ordering, windowing, cooldown reasoning, and evidence correlation.
+- Treat the current `timestamp` input column as `event_time` unless a config names another column through `time.timestamp_col`.
+- Preserve `observed_time` when a source provides it, but keep it separate from event ordering.
+- If a downstream export format only supports one timestamp, use `event_time` when present; fall back to `observed_time` only when the source event time is unavailable.
+- Derive `window_start` and `window_end` from event time, not from artifact generation time.
+- Use `artifact_generated_at` only for provenance and reproducibility checks.
+
+## Why It Matters
+
+Detection workflows become hard to review when ingestion time, source event time, analysis-window time, and report-generation time collapse into one ambiguous `timestamp`. This repository keeps those concepts explicit:
+
+- raw event records carry source event time
+- feature rows and alerts carry deterministic window boundaries
+- reports may carry artifact generation metadata
+- reviewer evidence can be checked without guessing which clock controlled the decision
+
+## References
+
+- [OpenTelemetry Logs Data Model](https://opentelemetry.io/docs/specs/otel/logs/data-model/)
diff --git a/tests/test_event_time_model_docs.py b/tests/test_event_time_model_docs.py
new file mode 100644
index 0000000..ce83d8a
--- /dev/null
+++ b/tests/test_event_time_model_docs.py
@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
+
+def _read_repo_file(relative_path: str) -> str:
+    return (REPO_ROOT / relative_path).read_text(encoding="utf-8")
+
+
+def test_event_time_model_documents_time_field_boundaries() -> None:
+    doc = _read_repo_file("docs/event-time-model.md")
+    readme = _read_repo_file("README.md")
+    docs_index = _read_repo_file("docs/README.md")
+
+    for term in [
+        "event_time",
+        "observed_time",
+        "window_start",
+        "window_end",
+        "artifact_generated_at",
+    ]:
+        assert term in doc
+
+    assert "OpenTelemetry Logs Data Model" in doc
+    assert "Timestamp" in doc
+    assert "ObservedTimestamp" in doc
+    assert "[window_start, window_end)" in doc
+    assert "must not be used as event evidence" in doc
+    assert "(docs/event-time-model.md)" in readme
+    assert "(event-time-model.md)" in docs_index