Fix Timestamp.from_datetime returning wrong value for pre-epoch datetimes

[bysiber]

Timestamp.from_datetime() uses int(dt.timestamp()) to compute the seconds component, but int() truncates towards zero. For pre-epoch datetimes with non-zero microseconds, this produces the wrong value:

import datetime
dt = datetime.datetime(1969, 12, 31, 23, 59, 59, 500000, tzinfo=datetime.timezone.utc)
# dt.timestamp() == -0.5
# int(-0.5) == 0  (truncation towards zero)
# Expected: Timestamp(seconds=-1, nanoseconds=500000000)
# Actual:   Timestamp(seconds=0, nanoseconds=500000000)  <-- +0.5s instead of -0.5s!

The sign of the time gets flipped. from_unix() already handles this correctly using floor division (int(unix_sec // 1)). This change makes from_datetime() consistent.

[methane]

@bysiber would you add a testcase for the fix to test_timestamp.py?

[Copilot]

Pull request overview

Fixes incorrect Timestamp.from_datetime() results for pre-epoch datetimes with fractional seconds by ensuring the seconds component is computed via flooring (consistent with Timestamp.from_unix()), avoiding int()’s truncation-toward-zero behavior.

Changes:

• Update Timestamp.from_datetime() to compute seconds using int(dt.timestamp() // 1) instead of int(dt.timestamp()).

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[methane]

@copilot apply changes based on this feedback