datetime - Python: Given the current time in UTC, how do you determine the start and end time of the day in a particular timezone?

Question

I'm playing around with Google App Engine and I learned that the timezone is fixed to UTC. I want to determine the start and end time of the current day for the user's local timezone. So basically, given the current time in UTC, how do you determine the start and end time of the current day, taking into account daylight savings changeovers.

I have some clunky example code. Please note that I realise that if I'm manually specifying a date, I could specify tomorrow's date as well, but they are examples and I want to determinte it programatically. My main problem is that if I add a timedelta to a datetime with a timezone and then normalise it (like it is suggested in the pytz docs) I get a datetime which is an hour off during daylight savings switchovers.

Not mentioned in the code, but ultimately the aim is to convert these times back to UTC which is why it's important to be timezone aware.

#!/usr/bin/python

import datetime
from pytz.gae import pytz

hobart_tz = pytz.timezone('Australia/Hobart')

utc_dt = pytz.utc.localize(datetime.datetime.utcnow())
hobart_dt = utc_dt.astimezone(hobart_tz)

# create a new datetime for the start of the day and add a day to it to get tomorrow.
today_start = datetime.datetime(hobart_dt.year, hobart_dt.month, hobart_dt.day)
today_start = hobart_tz.localize(today_start)
today_end = hobart_tz.normalize(today_start + datetime.timedelta(days=1))
print 'today:', today_start
print ' next:', today_end
print
# gives:
# today: 2011-08-28 00:00:00+10:00
# next: 2011-08-29 00:00:00+10:00

# but say today is a daylight savings changeover.
# after normalisation, we are off by an hour.

dst_finish_2011 = datetime.datetime(2011, 4, 3)  # this would come from hobart_dt
dst_finish_2011 = hobart_tz.localize(dst_finish_2011)
next = hobart_tz.normalize(dst_finish_2011 + datetime.timedelta(days=1))
print '2011-04-03:', dst_finish_2011
print '2011-04-04:', next   # expect 2011-04-04 00:00:00+10:00
print
# gives
# 2011-04-03: 2011-04-03 00:00:00+11:00
# 2011-04-04: 2011-04-03 23:00:00+10:00 (wrong)

dst_start_2011 = datetime.datetime(2011, 10, 2)  # this would come from hobart_dt
dst_start_2011 = hobart_tz.localize(dst_start_2011)
next = hobart_tz.normalize(dst_start_2011 + datetime.timedelta(days=1))
print '2011-10-02:', dst_start_2011
print '2011-10-03:', next   # expect 2011-10-03 00:00:00+11:00
print
# gives
# 2011-10-02: 2011-10-02 00:00:00+10:00
# 2011-10-03: 2011-10-03 01:00:00+11:00 (wrong)

# I guess we could ignore the timezone and localise *after* ?

dst_finish_2011 = datetime.datetime(2011, 4, 3)  # this would come from hobart_dt
next = dst_finish_2011 + datetime.timedelta(days=1)
# now localise
dst_finish_2011 = hobart_tz.localize(dst_finish_2011)
next = hobart_tz.localize(next)
print '2011-04-03:', dst_finish_2011
print '2011-04-04:', next   # expect 2011-04-04 00:00:00+10:00
print
# gives
# 2011-04-03: 2011-04-03 00:00:00+11:00
# 2011-04-04: 2011-04-04 00:00:00+10:00

Answer 1

To find out the start time of the day (midnight) and the end time of the day (tomorrow) in the local timezone, knowing UTC time:

#!/usr/bin/env python
from datetime import datetime, time, timedelta
import pytz # $ pip install pytz
from tzlocal import get_localzone # $ pip install tzlocal

tz = get_localzone() # get the local timezone as pytz.timezone
now = datetime.now(pytz.utc) # some UTC time
dt = now.astimezone(tz) # the same time in the local timezone
today = dt.date() # today in the local timezone (naive date object)
midnight = datetime.combine(today, time()) # midnight in the local timezone
aware_midnight = tz.localize(midnight, is_dst=None) # raise exception
                                                    # for ambiguous or
                                                    # non-existing
                                                    # times
tomorrow = midnight + timedelta(1)
aware_tomorrow = tz.localize(tomorrow, is_dst=None)

def print_time(aware_dt, fmt="%Y-%m-%d %H:%M:%S %Z%z"):
    print(aware_dt.strftime(fmt))
    utc_dt = aware_dt.astimezone(pytz.utc) # the same time in UTC
    print(utc_dt.strftime(fmt))

print_time(aware_midnight)
print_time(aware_tomorrow)

Output

2014-09-01 00:00:00 EST+1000
2014-08-31 14:00:00 UTC+0000
2014-09-02 00:00:00 EST+1000
2014-09-01 14:00:00 UTC+0000

See also,

• How do I get the UTC time of “midnight” for a given timezone? -- only midnight (no tomorrow)
• How can I subtract a day from a python date? -- about the difference between a day ago and yesterday
• python - datetime with timezone to epoch -- demonstrate how easy it is to produce a wrong answer (for some dates, timezones).