Django datetime.timedelta , how does its subtract from timezone.now() if they are posssibly different sets

Question

I am having trouble understanding functions that use:

time = timezone.now() - datetime.timedelta(days=30)

Firstly the timezone.now() gives the time that is set in Django … now the datetime.timedelta(days=30) …

does it use the internal settings in django as set setting.py or another.

Secondly if so … should: the variable time not be 30days behind the current timezone.now();

In the function below

def test_was_published_recently_with_old_question(self):
    """
    was_published_recently() should return False for questions whose
    pub_date is older than 1 day.
    """
    time = timezone.now() - datetime.timedelta(days=30)
    old_question = Question(pub_date=time)
    self.assertEqual(old_question.was_published_recently(), False)

how does this produce 1 day older … I think my issue is not understanding

time = timezone.now() - datetime.timedelta(days=30) in its entirely

I would really appreciate the help. I am new to python and med level programmer … but haven't worked much with time.

Answer 1

Firstly the timezone.now() gives the time that is set in Django … now the datetime.timedelta(days=30) …

does it use the internal settings in django as set setting.py or another.

First of all timezone.now() is just an "improved" version of datetime.datetime.now() that's also timezone aware.

timezone.now()
>>> datetime.datetime(2015, 9, 10, 19, 45, 34, 105121, tzinfo=<UTC>)
datetime.datetime.now()
>>> datetime.datetime(2015, 9, 10, 19, 45, 48, 356860)

There is no django internal time. timezone.now() is literally just the datetime function + timezone.

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timedelta then is just a difference, a delta as used in physics. It doesn't know anything about time.

When you substract/add a delta from a time you do the according thing to the time.

So today is 2015-09-10. If you add one day (timedelta(days=1)) to that that's obviously 2015-09-11.

A check whether something is more recent than a month in python is done by substracting 30 days from now (or 1 month if you like) and then comparing if the saved time is bigger than that.

---

To understand the datetime comparison better it may help converting these to unixtime and just seeing these abstract things as simple numbers by using the timestamp() function:

one_month_ago = (timezone.now() - datetime.timedelta(days=30)).timestamp()
now = timezone.now().timestamp()
print(one_month_ago)
print(now)
if one_month_ago < now:
    print("a month ago is smaller")

outputs

1439323463.786164
1441915463.786201
a month ago is smaller

Doing this without converting to timestamp prints a month ago is smaller too.