Moving a list of times forward a few months, taking into account daylight savings, and maybe a new timezone

timezone_sample.py

import datetime
import pytz
# Note pytz is not standard python library, but there
# are no standard timezone classes before Python 3.2!!
# This is Python 2.7 code.

source_timezone = "America/New_York"
destination_timezone = "America/New_York"

# There are 2 kinds of dates:
#   Naive dates just have date and time, and we have no idea what time zone they represent,
#       unless we know from context somehow
#   Timezoned dates have a time zone associated.  We can convert to other timezones, and
#       the hours (and sometimes minutes) change, but also the zone changes
#   We typically store dates timezoned, in UTC as the timezone.

# This exercise demos taking UTC dates for events that happened in Pittsburgh at midnight,
#   converting them to timezoned Pittsburgh dates, then making them naive, so
#   they all show up as midnight dates.
# Then we shift them forward a few weeks
# Then force them to be pittsburgh timezone-aware dates again,
# Then finally shift back to UTC to be standard.




# Suppose we have a list of module start and end dates; these are
#   stored internally as UTC.  If you call, for example
#   Course.objects.get(slug="f16-15319").start_time, you'll
#   get an object of type datetime.datetime, that is timezone-aware,
#   with timezone set to UTC

# Example has two pittsburgh-midnight dates before, and two after
# the time change
original_module_dates_utc = \
               [datetime.datetime(2020,1,1,5,0,0, tzinfo=pytz.utc),
                datetime.datetime(2020,1,7,5,0,0, tzinfo=pytz.utc),
                datetime.datetime(2020,7,1,4,0,0, tzinfo=pytz.utc),
                datetime.datetime(2020,7,7,4,0,0, tzinfo=pytz.utc)]

#
# Note: never do datetime(...tzinfo=pytz.<something other than UTC>); this will not handle dst correctly
#  see http://pytz.sourceforge.net/
#

print("Original dates as UTC")
for o in original_module_dates_utc:
    print(o)

#
# Convert from UTC to local time, naive
def utc2local_naive_tz(t, the_tz):
    t_naive = t.astimezone(pytz.timezone(the_tz))
    t_naive = t_naive.replace(tzinfo=None)
    return t_naive

print("Original dates as local, naive, assuming they happened in ", source_timezone)
original_dates_as_pgh_local_naive =  \
    [utc2local_naive_tz(t, source_timezone) for t in original_module_dates_utc]

for o in original_dates_as_pgh_local_naive:
    print(o)

#
# Offset the dates without affecting the times, so that
#  the translation ignores daylight savings changes

print("new dates as local, naive")
def addweeks(t,w):
    return t + datetime.timedelta(days=w*7)

new_dates_as_pgh_local_naive =  \
    [addweeks(t,25) for t in original_dates_as_pgh_local_naive]

for n in new_dates_as_pgh_local_naive:
    print(n)

#
# Convert back to UTC
#
def naive2utc_assuming_tz(t, dest):
    t_utc = pytz.timezone(dest).localize(t)
    t_utc = t_utc.astimezone(pytz.utc)
    return t_utc

new_dates_as_utc =  \
    [naive2utc_assuming_tz(t,destination_timezone)
           for t in new_dates_as_pgh_local_naive]

print("assuming those times were " + destination_timezone + ", convert back to utc")
for n in new_dates_as_utc:
    print(n)