diff --git a/docs/advisory_contract.md b/docs/advisory_contract.md
index 9f43fe1..e936c33 100644
--- a/docs/advisory_contract.md
+++ b/docs/advisory_contract.md
@@ -28,6 +28,20 @@ policy: object
 The current CLI and scheduled workflows produce v6. Readers continue to accept
 legacy v5 reports, but no v6 field is silently accepted on a v5 report.
 
+An explicit `as_of` is a UTC calendar date, not the invocation's local date.
+Its entire UTC day must have ended before a report can be generated:
+`generated_at >= reference_time`. For example, `as_of=2026-10-15` first becomes
+valid at `2026-10-16T00:00:00Z` (`2026-10-16T08:00:00+08:00`). An earlier build
+is rejected with the requested date, required UTC cutoff, and actual generation
+time. Offsets are normalized to UTC before comparison; equality is accepted.
+
+Leave the workflow's `as_of` input empty to select a fully closed period:
+yesterday for daily reports, the latest completed Saturday for weekly reports,
+or the last day of the previous month for monthly reports, all in UTC. A manual
+override keeps its requested date and is rejected if that date has not closed;
+the producer does not shift the cutoff or change generation timestamps to make
+an early request pass.
+
 ### V6 default contract
 
 A v6 report uses `contract_version = model_recommendations.v6` and adds these
diff --git a/src/quant_advisor_research/time_contract.py b/src/quant_advisor_research/time_contract.py
index 1211751..f8023fa 100644
--- a/src/quant_advisor_research/time_contract.py
+++ b/src/quant_advisor_research/time_contract.py
@@ -49,7 +49,10 @@ def report_time_bounds(as_of: dt.date, generated_at: dt.datetime | str) -> Repor
     reference_time = canonical_reference_time(as_of)
     generated = normalize_aware_datetime(generated_at)
     if generated < reference_time:
-        raise TimeContractError("generated_at must be at or after reference_time")
+        raise TimeContractError(
+            f"generated_at must be at or after reference_time: as_of={as_of.isoformat()} "
+            f"requires generated_at >= {reference_time.isoformat()}; got {generated.isoformat()}"
+        )
     return ReportTimeBounds(reference_time, generated, generated + dt.timedelta(days=REPORT_EXPIRY_DAYS))
 
 
diff --git a/tests/test_time_contract.py b/tests/test_time_contract.py
index f608e17..af36fe2 100644
--- a/tests/test_time_contract.py
+++ b/tests/test_time_contract.py
@@ -35,6 +35,30 @@ def test_reference_and_build_order_are_strict() -> None:
         report_time_bounds(dt.date(2026, 5, 30), "2026-05-30T12:00:00Z")
 
 
+@pytest.mark.parametrize(
+    "generated_at",
+    ["2026-10-15T23:59:59.999999Z", "2026-10-16T07:59:59.999999+08:00"],
+)
+def test_unclosed_as_of_reports_the_required_utc_cutoff(generated_at: str) -> None:
+    with pytest.raises(TimeContractError) as exc:
+        report_time_bounds(dt.date(2026, 10, 15), generated_at)
+
+    assert "as_of=2026-10-15" in str(exc.value)
+    assert "requires generated_at >= 2026-10-16T00:00:00+00:00" in str(exc.value)
+    assert "got 2026-10-15T23:59:59.999999+00:00" in str(exc.value)
+
+
+@pytest.mark.parametrize(
+    "generated_at",
+    ["2026-10-16T00:00:00Z", "2026-10-16T08:00:00+08:00", "2026-10-15T20:00:00-04:00"],
+)
+def test_closed_as_of_accepts_the_exact_cutoff_in_any_offset(generated_at: str) -> None:
+    bounds = report_time_bounds(dt.date(2026, 10, 15), generated_at)
+
+    assert bounds.reference_time == dt.datetime(2026, 10, 16, tzinfo=dt.UTC)
+    assert bounds.generated_at == bounds.reference_time
+
+
 def test_context_date_only_expiry_uses_original_offset_local_eod() -> None:
     assert context_expiry_instant("2026-06-30", "2026-05-30T00:30:00+08:00") == dt.datetime(
         2026, 6, 30, 15, 59, 59, tzinfo=dt.UTC