python daylight saving time issues

Question

I am having trouble with daylight saving time.

Let me explain the context.

I store dates in UTC in my database. Each user has a time zone and I store this information in the database as well.

I calculate periods of "1 month" for each user based on their time zone.

For example, the user should see:

2014-01-06 -> 2014-02-05
2014-02-06 -> 2014-03-05
2014-03-06 -> 2014-04-05
2014-04-06 -> 2014-05-05
2014-05-06 -> 2014-06-05
etc.

I have a problem when the Daylight Saving time goes on or off (March and November)

Instead of:

2014-03-06 -> 2014-04-05, I get 2014-03-06 -> 2014-04-06
2014-11-06 -> 2014-12-05, I get 2014-11-05 -> 2014-12-04

and then it's offset for all the next ones.

I tried to work with UTC dates instead of converting them to the user time zone to add 1 month in get_end_date but I still had issues.

Any idea how to handle this?

Thanks

Here is my code and the get_end_date method.

import pytz
from dateutil.parser import parse as parse_date
from dateutil.relativedelta import relativedelta

def to_tz(dt, tz):
    tz = pytz.timezone(tz)
    return tz.normalize(dt.astimezone(tz))

def get_end_date(dt, tz):
    dt_central = to_tz(dt, tz)

    # Add 1 month, set hours, minutes, seconds, microseconds to 0 and remove -1 ms
    dt_central = dt_central + relativedelta(months=+1)
    dt_central = dt_central.replace(hour=0, minute=0, second=0, microsecond=0)
    dt_central = dt_central + relativedelta(microseconds=-1)

    return to_tz(dt_central, 'UTC')

def print_periods(start_dt, tz, nb_periods=6):    
    for i in xrange(nb_periods):
        print '\n# Period {}'.format(i+1)

        end_dt = get_end_date(start_dt, tz)
        start_dt_central, end_dt_central = to_tz(start_dt, tz), to_tz(end_dt, tz)

        print '{} -> {}'.format(start_dt_central.date(), end_dt_central.date())
        print '  > start_dt_central: ', start_dt_central
        print '  > end_dt_central: ', end_dt_central
        print '  > start_dt: ', start_dt
        print '  > end_dt: ', end_dt

        start_dt = end_dt + relativedelta(microseconds=1)

The outputs:

central_tz = 'US/Central'
start_dt = parse_date('2014-01-06 06:00:00+00:00')

In [7]: print_periods(start_dt, central_tz, nb_periods=12)
# Period 1
2014-01-06 -> 2014-02-05
  > start_dt_central:  2014-01-06 00:00:00-06:00
  > end_dt_central:  2014-02-05 23:59:59.999999-06:00
# Period 2
2014-02-06 -> 2014-03-05
  > start_dt_central:  2014-02-06 00:00:00-06:00
  > end_dt_central:  2014-03-05 23:59:59.999999-06:00
# Period 3
2014-03-06 -> 2014-04-06
  > start_dt_central:  2014-03-06 00:00:00-06:00
  > end_dt_central:  2014-04-06 00:59:59.999999-05:00
# Period 4
2014-04-06 -> 2014-05-05
  > start_dt_central:  2014-04-06 01:00:00-05:00
  > end_dt_central:  2014-05-05 23:59:59.999999-05:00
# Period 5
2014-05-06 -> 2014-06-05
  > start_dt_central:  2014-05-06 00:00:00-05:00
  > end_dt_central:  2014-06-05 23:59:59.999999-05:00
# Period 6
2014-06-06 -> 2014-07-05
  > start_dt_central:  2014-06-06 00:00:00-05:00
  > end_dt_central:  2014-07-05 23:59:59.999999-05:00
# Period 7
2014-07-06 -> 2014-08-05
  > start_dt_central:  2014-07-06 00:00:00-05:00
  > end_dt_central:  2014-08-05 23:59:59.999999-05:00
# Period 8
2014-08-06 -> 2014-09-05
  > start_dt_central:  2014-08-06 00:00:00-05:00
  > end_dt_central:  2014-09-05 23:59:59.999999-05:00
# Period 9
2014-09-06 -> 2014-10-05
  > start_dt_central:  2014-09-06 00:00:00-05:00
  > end_dt_central:  2014-10-05 23:59:59.999999-05:00
# Period 10
2014-10-06 -> 2014-11-05
  > start_dt_central:  2014-10-06 00:00:00-05:00
  > end_dt_central:  2014-11-05 22:59:59.999999-06:00
# Period 11
2014-11-05 -> 2014-12-04
  > start_dt_central:  2014-11-05 23:00:00-06:00
  > end_dt_central:  2014-12-04 23:59:59.999999-06:00
# Period 12
2014-12-05 -> 2015-01-04
  > start_dt_central:  2014-12-05 00:00:00-06:00
  > end_dt_central:  2015-01-04 23:59:59.999999-06:00

UPDATE: SOLUTIONS

Solution 1 from Christian

def get_end_date(dt, tz):
    dt_central = to_tz(dt, tz)

    orig_tz = dt_central.tzinfo
    dt_central = dt_central.replace(tzinfo=None)

    # Calculations

    dt_central = orig_tz.localize(dt_central)

    dt_utc = to_tz(dt_central, 'UTC')

    return dt_utc

Solution 2 from myself

def get_end_date(dt, tz):
    dt_central = to_tz(dt, tz)

    # Calculations

    dt_utc = to_tz(dt_central, 'UTC')

    # When switch to Daylight Saving Time
    if to_tz(dt_utc, tz).hour == 0:
        dt_utc = dt_utc.replace(hour=dt_utc.hour-1)
    # When leave Daylight Saving Time  
    elif to_tz(dt_utc, tz).hour == 22:
        dt_utc = dt_utc.replace(hour=dt_utc.hour+1)

    return dt_utc

Christian Geier Christian Geier · Accepted Answer · 2014-09-08T13:15:50

I had a similar problem with dateutil's rrule while expanding vevents' reccurrence rules. What worked for me was saving the timezone information, then transforming each datetime object into a naive datetime object, do the calculations and than localize each datetime with the saved timezone.

So something along those lines:

orig_tz = dt.tzinfo
dt = dt.replace(tzinfo=None)

# do calculations here

orig_tz.localize(dt)

python daylight saving time issues

UPDATE: SOLUTIONS

1 Answers