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Fix naturaltime comparisons across daylight-saving transitions #425

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Fix naturaltime comparisons across daylight-saving transitions #425

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b5b5694
fix: compare aware naturaltime inputs as UTC instants
pralav-25 Oct 1, 2026
327207a
test: cover same-zone daylight saving transitions
pralav-25 Oct 13, 2026
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22 changes: 11 additions & 11 deletions src/humanize/time.py
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Original file line number Diff line number Diff line change
Expand Up @@ -290,10 +290,13 @@ def naturaltime(
"""
import datetime as dt

value = _convert_aware_datetime(value)
when = _convert_aware_datetime(when)

now = when or _now()
aware_value = isinstance(value, dt.datetime) and value.tzinfo is not None
now = when or (dt.datetime.now(dt.timezone.utc) if aware_value else _now())
if aware_value or now.tzinfo is not None:
# Compare instants in UTC so a local daylight-saving transition cannot
# erase a repeated hour or count a skipped hour twice.
value = _convert_datetime_to_utc(value)
now = _convert_datetime_to_utc(now)

date, delta = _date_and_delta(value, now=now)
if date is None:
Expand All @@ -311,17 +314,14 @@ def naturaltime(
return str(ago % delta)


def _convert_aware_datetime(
def _convert_datetime_to_utc(
value: dt.datetime | dt.timedelta | float | None,
) -> Any:
"""Convert aware datetime to naive datetime and pass through any other type."""
if value is None:
return None

"""Convert a datetime to naive UTC and pass through any other type."""
import datetime as dt

if isinstance(value, dt.datetime) and value.tzinfo is not None:
value = dt.datetime.fromtimestamp(value.timestamp())
if isinstance(value, dt.datetime):
value = value.astimezone(dt.timezone.utc).replace(tzinfo=None)
return value


Expand Down
97 changes: 97 additions & 0 deletions tests/test_time.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@

import datetime as dt
import typing
from zoneinfo import ZoneInfo, ZoneInfoNotFoundError

import pytest
from freezegun import freeze_time
Expand Down Expand Up @@ -567,6 +568,102 @@ def test_naturaltime_timezone_when(test_input: dt.datetime, expected: str) -> No
assert humanize.naturaltime(test_input, when=NOW_UTC) == expected


@freeze_time("2026-11-01 06:30:00")
@pytest.mark.parametrize(
"value, when, expected",
[
(
dt.datetime(2026, 11, 1, 1, 30, tzinfo=dt.timezone(dt.timedelta(hours=-4))),
dt.datetime(2026, 11, 1, 1, 30, tzinfo=dt.timezone(dt.timedelta(hours=-5))),
"an hour ago",
),
(
dt.datetime(2026, 11, 1, 1, 30, tzinfo=dt.timezone(dt.timedelta(hours=-5))),
dt.datetime(2026, 11, 1, 1, 30, tzinfo=dt.timezone(dt.timedelta(hours=-4))),
"an hour from now",
),
(
dt.datetime(2026, 3, 8, 1, 30, tzinfo=dt.timezone(dt.timedelta(hours=-5))),
dt.datetime(2026, 3, 8, 3, 30, tzinfo=dt.timezone(dt.timedelta(hours=-4))),
"an hour ago",
),
(
dt.datetime(2026, 11, 1, 1, 30, tzinfo=dt.timezone(dt.timedelta(hours=-4))),
dt.datetime(2026, 11, 1, 1, 30, fold=1),
"an hour ago",
),
(
dt.datetime(2026, 11, 1, 1, 30),
dt.datetime(2026, 11, 1, 1, 30, tzinfo=dt.timezone(dt.timedelta(hours=-5))),
"an hour ago",
),
(
dt.datetime(2026, 11, 1, 1, 30, tzinfo=dt.timezone(dt.timedelta(hours=-4))),
None,
"an hour ago",
),
],
)
def test_naturaltime_daylight_saving(
value: dt.datetime,
when: dt.datetime | None,
expected: str,
monkeypatch: pytest.MonkeyPatch,
) -> None:
import time as system_time

if not hasattr(system_time, "tzset"):
pytest.skip("Changing the local timezone requires time.tzset")
try:
with monkeypatch.context() as timezone:
timezone.setenv("TZ", "EST5EDT,M3.2.0,M11.1.0")
system_time.tzset()
assert humanize.naturaltime(value, when=when) == expected
finally:
system_time.tzset()


@pytest.mark.parametrize(
"value, when",
[
(
dt.datetime(2026, 11, 1, 1, 30, fold=0),
dt.datetime(2026, 11, 1, 1, 30, fold=1),
),
(
dt.datetime(2026, 3, 8, 1, 30),
dt.datetime(2026, 3, 8, 3, 30),
),
],
)
def test_naturaltime_daylight_saving_same_zone(
value: dt.datetime, when: dt.datetime, monkeypatch: pytest.MonkeyPatch
) -> None:
import time as system_time

if not hasattr(system_time, "tzset"):
pytest.skip("Changing the local timezone requires time.tzset")

try:
eastern = ZoneInfo("America/New_York")
except ZoneInfoNotFoundError:
pytest.skip("IANA timezone database is unavailable")

value = value.replace(tzinfo=eastern)
when = when.replace(tzinfo=eastern)

try:
with monkeypatch.context() as timezone:
timezone.setenv("TZ", "EST5EDT,M3.2.0,M11.1.0")
system_time.tzset()
assert value.tzinfo is when.tzinfo
assert abs(value.timestamp() - when.timestamp()) == ONE_HOUR
assert humanize.naturaltime(value, when=when) == "an hour ago"
assert humanize.naturaltime(when, when=value) == "an hour from now"
finally:
system_time.tzset()


@pytest.mark.parametrize(
"val, min_unit, expected",
[
Expand Down