django.utils.timesince() does not account for leap years

[Jannis Vajen]

The output of django.utils.timesince() will become increasingly inaccurate as the number of leap years in the supplied time period grows.

[Claude Paroz]

Using 365.2425 instead of 365 in chunks might give better results for long periods, without much distortion for shorter periods.

[Hiroki Kiyohara]

Added test to verifiy the behavior while long time.

[Hiroki Kiyohara]

I applied my patch and tried test, using 365.2425 instead of 365 in chunks.
test_thousand_years_ago passed, but a test failed, like this:

======================================================================
FAIL: test_other_units (regressiontests.utils.timesince.TimesinceTests)
Test other units.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/hirokiky/django/django/tests/regressiontests/utils/timesince.py", line 42, in test_other_units
    self.assertEqual(timesince(self.t, self.t+self.oneyear), '1 year')
AssertionError: u'12 months' != u'1 year'
- 12 months
+ 1 year


----------------------------------------------------------------------

[Hiroki Kiyohara]

Fixed test not to use dateutil ​https://github.com/hirokiky/django/commit/b3b81594fc6c0a424c414ffdc751e01c55e11cb1
Because, other sources doesn't use it.

[Hiroki Kiyohara]

Solved with dirty implemantion ​https://github.com/hirokiky/django/commit/f581659080e78891d043ecbd63e2fc349c786c44
And commited to fix this commit ​https://github.com/hirokiky/django/commit/68d64fe91a73dbf4bebda98ebee0bb7b8b084ab6

It seems good to separate the resolusion depending on whether the unit is year or not.

[Hiroki Kiyohara]

I send pull-request as ​https://github.com/django/django/pull/622 .

Please check it, if you do not mind.

[Susan Tan]

@hirokiky, I've applied your PR branch and there are errors in the utils_tests test suite. See stack trace below:

======================================================================
FAIL: test_date_objects (utils_tests.test_timesince.TimesinceTests)
Both timesince and timeuntil should work on date objects (#17937).

---

Traceback (most recent call last):
../Projects/django/tests/utils_tests/test_timesince.py", line 112, in test_date_objects

self.assertEqual(timesince(today + self.oneday), '0\xa0minutes')

AssertionError: u'0 minutes' != u'0\xa0minutes'

• 0 minutes

?
^{+ 0\xa0minutes
?}

======================================================================
FAIL: test_different_timezones (utils_tests.test_timesince.TimesinceTests)
When using two different timezones.

---

Traceback (most recent call last):
../Projects/django/tests/utils_tests/test_timesince.py", line 105, in test_different_timezones

self.assertEqual(timesince(now), '0\xa0minutes')

AssertionError: u'0 minutes' != u'0\xa0minutes'

• 0 minutes

?
^{+ 0\xa0minutes
?}

======================================================================
FAIL: test_display_second_before_first (utils_tests.test_timesince.TimesinceTests)

---

Traceback (most recent call last):
../Projects/django/tests/utils_tests/test_timesince.py", line 74, in test_display_second_before_first

'0\xa0minutes')

AssertionError: u'0 minutes' != u'0\xa0minutes'

• 0 minutes

?
^{+ 0\xa0minutes
?}

======================================================================
FAIL: test_equal_datetimes (utils_tests.test_timesince.TimesinceTests)
equal datetimes.

---

Traceback (most recent call last):
../Projects/django/tests/utils_tests/test_timesince.py", line 25, in test_equal_datetimes

self.assertEqual(timesince(self.t, self.t), '0\xa0minutes')

AssertionError: u'0 minutes' != u'0\xa0minutes'

• 0 minutes

?
^{+ 0\xa0minutes
?}

======================================================================
FAIL: test_ignore_microseconds_and_seconds (utils_tests.test_timesince.TimesinceTests)
Microseconds and seconds are ignored.

---

Traceback (most recent call last):
../Projects/django/tests/utils_tests/test_timesince.py", line 30, in test_ignore_microseconds_and_seconds

'0\xa0minutes')

AssertionError: u'0 minutes' != u'0\xa0minutes'

• 0 minutes

?
^{+ 0\xa0minutes
?}

======================================================================
FAIL: test_naive_datetime_with_tzinfo_attribute (utils_tests.test_timesince.TimesinceTests)

---

Traceback (most recent call last):
.../django/tests/utils_tests/test_timesince.py", line 127, in test_naive_datetime_with_tzinfo_attribute

self.assertEqual(timesince(future), '0\xa0minutes')

AssertionError: u'0 minutes' != u'0\xa0minutes'

• 0 minutes

?
^{+ 0\xa0minutes
?}

---

Ran 248 tests in 0.473s

FAILED (failures=6, errors=6)

Did anyone else also get errors? Or did all the tests pass?

[Tim Graham]

Susan, it sounds like you may be running the latest version of the tests with a version of Django before 7d77e9786a118dd95a268872dd9d36664066b96a (or perhaps you didn't apply the pull request quite right).

[Susan Tan]

The 6-mo old patch was created before that new commit you're referring to, timo. That means the patch needs to be updated.

[Susan Tan]

I partially updated the patch on my origin branch here: ​https://github.com/onceuponatimeforever/django/compare/pull_622. It's not a PR, because there are still errors. I say "partially updated", because when I run the utils_tests, I get a dozen errors related to this:

 File "../django/django/utils/timesince.py", line 65, in timesince
    s = ugettext('%(number)d %(type)s') % {'number': count, 'type': name(count)}
TypeError: '__proxy__' object is not callable

I'm not sure how to fix this error. But I'll leave it up to the next contributor to take a look at this.

[Hiroki Kiyohara]

Thanks @susan @timo. I can't take time for this problem, so I'm leaving.

[Aymeric Augustin]

For what it's worth, I consider that timesince returns an order of magnitude for human consumption and a small inaccuracy isn't critical. I wouldn't mind if this ticket was closed as wontfix.

[Jannis Vajen]

What about the use case that I want to display the age of a person given his or her birthdate? I consider this an order of magnitude for human consumption but timesince will be displaying an incorrect number of years.

[Raphael Michel]

After looking into a lot of algorithms for date diffing, including the ones suggested above and the ones from python-dateutil (which get pretty complex) I decided to go for the most simple solution I could find. I assume timesince wasn't ever meant to be an exact calculation. If we would want an exact calculation, we should completely re-implement it to correctly work with the different number of days in the months and so on.

However, there is a very simple fix to the accumulated error that grows with the given time period: just use calendar.leapdays from the standard library and substract it from the number of days.

I created a pull request including a regression test: ​https://github.com/django/django/pull/4785

[Tim Graham <timograham@…>]

In 6700c90:

Fixed #19210 -- Added leap year support to django.utils.timesince()