python whatwg How do I get the UTC time of “midnight” for a given timezone?




whatwg developers (4)

The best I can come up with for now is this monstrosity:

>>> datetime.utcnow() \
...   .replace(tzinfo=pytz.UTC) \
...   .astimezone(pytz.timezone("Australia/Melbourne")) \
...   .replace(hour=0,minute=0,second=0,microsecond=0) \
...   .astimezone(pytz.UTC) \
...   .replace(tzinfo=None)
datetime.datetime(2008, 12, 16, 13, 0)

I.e., in English, get the current time (in UTC), convert it to some other timezone, set the time to midnight, then convert back to UTC.

I'm not just using now() or localtime() as that would use the server's timezone, not the user's timezone.

I can't help feeling I'm missing something, any ideas?


@hop's answer is wrong on the day of transition from Daylight Saving Time (DST) e.g., Apr 1, 2012. To fix it tz.localize() could be used:

tz = pytz.timezone("Australia/Melbourne")
today = datetime.now(tz).date()
midnight = tz.localize(datetime.combine(today, time(0, 0)), is_dst=None)
utc_dt = midnight.astimezone(pytz.utc)        

The same with comments:

#!/usr/bin/env python
from datetime import datetime, time
import pytz # pip instal pytz

tz = pytz.timezone("Australia/Melbourne") # choose timezone

# 1. get correct date for the midnight using given timezone.
today = datetime.now(tz).date()

# 2. get midnight in the correct timezone (taking into account DST)
#NOTE: tzinfo=None and tz.localize()
# assert that there is no dst transition at midnight (`is_dst=None`)
midnight = tz.localize(datetime.combine(today, time(0, 0)), is_dst=None)

# 3. convert to UTC (no need to call `utc.normalize()` due to UTC has no 
#    DST transitions)
fmt = '%Y-%m-%d %H:%M:%S %Z%z'
print midnight.astimezone(pytz.utc).strftime(fmt)

I think you can shave off a few method calls if you do it like this:

>>> from datetime import datetime
>>> datetime.now(pytz.timezone("Australia/Melbourne")) \
            .replace(hour=0, minute=0, second=0, microsecond=0) \
            .astimezone(pytz.utc)

BUT… there is a bigger problem than aesthetics in your code: it will give the wrong result on the day of the switch to or from Daylight Saving Time.

The reason for this is that neither the datetime constructors nor replace() take DST changes into account.

For example:

>>> now = datetime(2012, 4, 1, 5, 0, 0, 0, tzinfo=pytz.timezone("Australia/Melbourne"))
>>> print now
2012-04-01 05:00:00+10:00
>>> print now.replace(hour=0)
2012-04-01 00:00:00+10:00 # wrong! midnight was at 2012-04-01 00:00:00+11:00
>>> print datetime(2012, 3, 1, 0, 0, 0, 0, tzinfo=tz)
2012-03-01 00:00:00+10:00 # wrong again!

However, the documentation for tz.localize() states:

This method should be used to construct localtimes, rather than passing a tzinfo argument to a datetime constructor.

Thus, your problem is solved like so:

>>> import pytz
>>> from datetime import datetime, date, time

>>> tz = pytz.timezone("Australia/Melbourne")
>>> the_date = date(2012, 4, 1) # use date.today() here

>>> midnight_without_tzinfo = datetime.combine(the_date, time())
>>> print midnight_without_tzinfo
2012-04-01 00:00:00

>>> midnight_with_tzinfo = tz.localize(midnight_without_tzinfo)
>>> print midnight_with_tzinfo
2012-04-01 00:00:00+11:00

>>> print midnight_with_tzinfo.astimezone(pytz.utc)
2012-03-31 13:00:00+00:00

No guarantees for dates before 1582, though.


Setting the TZ environment variable modifies what timezone Python's date and time functions work with.

>>> time.gmtime()
(2008, 12, 17, 1, 16, 46, 2, 352, 0)
>>> time.localtime()
(2008, 12, 16, 20, 16, 47, 1, 351, 0)
>>> os.environ['TZ']='Australia/Melbourne'
>>> time.localtime()
(2008, 12, 17, 12, 16, 53, 2, 352, 1)

Each time zone has a number, eg US/Central = -6. This is defined as the offset in hours from UTC. Since 0000 is midnight, you can simply use this offset to find the time in any time zone when it is midnight UTC. To access that, I believe you can use

 time.timezone

According to The Python Docs, time.timezone actually gives the negative value of this number:

time.timezone

The offset of the local (non-DST) timezone, in seconds west of UTC (negative in most of Western Europe, positive in the US, zero in the UK).

So you would simply use that number for the time in hours if it's positive (i.e., if it's midnight in Chicago (which has a +6 timezone value), then it's 6000 = 6am UTC).

If the number is negative, subtract from 24. For example, Berlin would give -1, so 24 - 1 => 2300 = 11pm.





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