How do I get the UTC time of "midnight" for a given timezone?

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?

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.

@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)

This is more straightforward with dateutil.tz than pytz:

>>>import datetime
>>>import dateutil.tz
>>>midnight=(datetime.datetime
             .now(dateutil.tz.gettz('Australia/Melbourne'))
             .replace(hour=0, minute=0, second=0, microsecond=0)
             .astimezone(dateutil.tz.tzutc()))
>>>print(midnight)
2019-04-26 14:00:00+00:00

The tzinfo documentation recommends dateutil.tz since Python 3.6. The tzinfo objects from dateutil.tz have no problems with anomalies like DST without requiring the localize functionality of pytz. Using the example from user3850:

>>> now = (datetime.datetime(2012, 4, 1, 5,  
...         tzinfo = dateutil.tz.gettz('Australia/Melbourne'))) 
>>> print(now.replace(hour = 0).astimezone(dateutil.tz.tzutc()))
2012-03-31 13:00:00+00:00

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.

It's worth remarking that we can adapt the answer given by @jfs to find tomorrow's midnight or yesterday's midnight, etc. The trick is to add a certain number of days to the aware timezone. This works because although this usually adds 24 hours, sometimes it might add 23 or 25 based on DST issues.

from datetime import datetime, time, timedelta
import pytz

def midnight_UTC(offset):

    # Construct a timezone object
    tz = pytz.timezone('Australia/Melbourne')

    # Work out today/now as a timezone-aware datetime
    today = datetime.now(tz)

    # Adjust by the offset. Note that that adding 1 day might actually move us 23 or 25
    # hours into the future, depending on daylight savings. This works because the {today}
    # variable is timezone aware
    target_day = today + timedelta(days=1) * offset

    # Discard hours, minutes, seconds and microseconds
    midnight_aware = tz.localize(
        datetime.combine(target_day, time(0, 0, 0, 0)), is_dst=None)

    # Convert to UTC
    midnight_UTC = midnight_aware.astimezone(pytz.utc)

    return midnight_UTC

print("The UTC time of the previous midnight is:", midnight_UTC(0))
print("The UTC time of the upcoming midnight is:", midnight_UTC(1))