django.utils.timesince.timesince incorrectly handles daylight saving time

[Frank Sauerburger]

timesince computes the time elapsed between two datetimes (d and now) and returns it as a human readable string. The function is intended to show the elapsed time from a user perspective (sitting with a stopwatch in front of the computer). timesince relies on Python's timezone arithmetic, however, there are subtle implementation details for intra- and inter-timezone calculations. See

• Discussion about the topic: ​https://github.com/python/cpython/issues/116111
• Quiz to illustrate subtle examples: ​https://quiz.sauerburger.com/dxi7m/

Consider the following example around the daylight saving time transition next weekend in Europe. We start at a point in time a, ten minutes later we have a_10, and another 60 minutes later we have a_70.

from zoneinfo import ZoneInfo
from datetime import datetime
from django.utils.timesince import timesince

berlin = ZoneInfo("Europe/Berlin")

a = datetime(2024, 10, 27, 2, 55, tzinfo=berlin)
a_10 = datetime(2024, 10, 27, 2, 5, fold=1, tzinfo=berlin)
a_70 = datetime(2024, 10, 27, 3, 5, tzinfo=berlin)

assert a.isoformat() == '2024-10-27T02:55:00+02:00'
assert a_10.isoformat() == '2024-10-27T02:05:00+01:00'
assert a_70.isoformat() == '2024-10-27T03:05:00+01:00'

assert timesince(a, a_10) == '0\xa0minutes'
assert timesince(a, a_70) == '10\xa0minutes'

My expectation is that timesince(a, a_10) yields 10 minutes and timesince(a, a_70) yields 70 minutes aligned with what a user with a stopwatch would observe.

I think this can lead to a lot of unexpected behavior in web applications around the DST transition and maybe even exploitable behavior.

[Sarah Boyce]

If I understood correctly, I think the example was meant to show:

a = datetime(2024, 10, 27, 2, 55, tzinfo=berlin)
a_10 = datetime(2024, 10, 27, 3, 5, fold=0, tzinfo=berlin)
a_70 = datetime(2024, 10, 27, 3, 5, fold=1, tzinfo=berlin)
assert timesince(a, a_10) == '10\xa0minutes'
assert timesince(a, a_70) == '10\xa0minutes'  # expected 1 hour 10 minutes

For those not familiar with ​fold, this is used to disambiguate wall times during a repeated interval. The values 0 and 1 represent, respectively, the earlier and later of the two moments with the same wall time representation.

Looking at the ​discussion, it appears timesince should convert the dates to UTC before subtracting them.

Linking #34483 as this is slightly related

[Frank Sauerburger]

Hi Sarah,

yes that's also a good way to demonstrate the issue. In my example, I also wanted to demonstrate that timesince can confuse the order of events. a_10 happens 10 minutes after event a, but timesince returns '0 minutes' as if the order was reversed.

EDIT: Actually, if I'm not mistaken, in your example, there is no difference between

a_10 = datetime(2024, 10, 27, 3, 5, fold=0, tzinfo=berlin)
a_70 = datetime(2024, 10, 27, 3, 5, fold=1, tzinfo=berlin)

The overlapping folds are between 2 and 3 am.

[Sarah Boyce]

Ah I see, sorry Frank I misunderstood your example