如何获取本地时区datetime以进行Django中的日期比较

Question

I am trying to fire a reminder based on a users timezone. The users timezone is 'America/New_York'

I attempt to get the users time with the following:

now = datetime.now()
now_plus_10 = test_customer.time_zone.localize(now + timedelta(minutes = 10))
now_minus_10 = test_customer.time_zone.localize(now + timedelta(minutes = -10))

The reminder time is at 13:00. It is gathered from the customer object like this:

 reminder = customer.reminder #set to 13:00

I then want to send a reminder if within 10 minutes + or - of the reminder.

I do the comparison like:

if reminder > now_minus_10 and gb_real_time < now_plus_10: 
    reminder_email.send()
else:
    print 'ceiling '+str(now_plus_10)
    print 'not in window don t send'
    print 'floor '+str(now_minus_10)

This prints:

ceiling 2014-09-03 10:10:54.547901-04:00  
not in window don t send
floor 2014-09-03 09:50:54.547901-04:00

I am in the timezone: 'America/Los_Angeles' in which it is 10:00am

Why is it that even though I localized now_plus_10 and now_minus_10 dates, but they do not convert to New_York time which would make them 13:00 like the reminder and in turn fire the reminder?

Answer 1

I think you've misunderstood what localize() does. (I assume you're talking about the pytz localize() , in which case you can see the documentation here .)

It doesn't convert times, it takes a naive (timezone-unaware) datetime and makes it aware ( assigns it a timezone). Here's what's going on in your example:

now = datetime.now()  # 10:00am local time (no timezone)
later = now + timedelta(minutes=10)  # 10:10am (no timezone)
now_plus_10 = ny_timezone.localize(later)  # 10:10am (NY timezone)

If you really want to convert between timezones directly, using pytz , you can do something like this:

now = la_timzeone.localize(datetime.now())  # 10:00am local time (LA timezone)
later = now + timedelta(minutes=10)  # 10:10am (LA timezone)
later_safe = la_timezone.normalize(later)  # because of DST and such
later_ny = later_safe.astimezone(ny_timezone)  # 13:10 (NY timezone)
later_ny_safe = ny_timezone.normalize(later_ny)  # again

Now, that's complicated! Which is why both pytz and Django discourage you from manually localizing and converting time zones. You'll have to review the documentation , but basically the Django approach is this:

• All internal datetimes are timezone-aware and use UTC.
• Use django.utils.timezone.now() to get the current time in UTC.
• User-entered datetimes (for example, if your user enters his or her reminder time in a form) are automatically made aware. To do this you have to know the user's timezone and then call activate() . You can also do it manually with localtime() .
• Do all of your comparisons and arithmetic with these UTC datetimes.

I definitely recommend doing things the Django way.