fix: keep exact milliseconds in from_datetime and from_timestamp

CHANGELOG.rst

@@ -8,6 +8,11 @@ Versions follow `Semantic Versioning <http://www.semver.org>`_
 `Unreleased`_
 -------------
 
+Fixed
+~~~~~
+* :meth:`.ULID.from_datetime` and :meth:`.ULID.from_timestamp` no longer land one millisecond early
+  when a value on an exact millisecond multiplies to just below that integer as a float.
+
 `4.1.0`_ - 2026-10-27
 ---------------------
 Changed

tests/test_ulid.py

@@ -337,6 +337,19 @@ def test_ulid_from_timestamp() -> None:
     assert ulid1.timestamp == ulid2.timestamp
 
 
+@pytest.mark.parametrize("milliseconds", [1078625331860, 1097375394952])
+def test_ulid_from_exact_milliseconds(milliseconds: int) -> None:
+    # Both values multiply back to just below the integer: x.860 * 1000 == x859.9999...
+    seconds = milliseconds / constants.MILLISECS_IN_SECS
+    assert ULID.from_timestamp(seconds).milliseconds == milliseconds
+    assert ULID.from_datetime(datetime.fromtimestamp(seconds, UTC)).milliseconds == milliseconds
+
+
+def test_ulid_timestamp_keeps_truncating_sub_milliseconds() -> None:
+    expected = 1000999
+    assert ULID.from_timestamp(1000.9999).milliseconds == expected
+
+
 def test_ulid_from_timestamp_overflow() -> None:
     with pytest.raises(ValueError, match="Value exceeds maximum possible timestamp"):
         ULID.from_timestamp(constants.MAX_TIMESTAMP + 1)

ulid/__init__.py

@@ -188,13 +188,18 @@ def _default_clock() -> int:
     def _default_randomness(_timestamp: int) -> bytes:
         return os.urandom(constants.RANDOMNESS_LEN)
 
+    @staticmethod
+    def _seconds_to_milliseconds(seconds: float) -> int:
+        # Round off the float error first, or x.860 s truncates to x859 ms.
+        return int(round(seconds * constants.MILLISECS_IN_SECS, 3))
+
     def _normalize_timestamp(self, value: float | datetime | None = None) -> int:
         if value is None:
             value = self.clock()
         elif isinstance(value, datetime):
-            value = int(value.timestamp() * constants.MILLISECS_IN_SECS)
+            value = self._seconds_to_milliseconds(value.timestamp())
         elif isinstance(value, float):
-            value = int(value * constants.MILLISECS_IN_SECS)
+            value = self._seconds_to_milliseconds(value)
         if value > constants.MAX_TIMESTAMP:
             raise ValueError("Value exceeds maximum possible timestamp")
         return value