How do I know if today is a day due to change civil local time e.g. daylight saving time in standard python and pandas timestamps? - python-3.x

Question

According to the rules of British Summer Time / daylight saving time (https://www.gov.uk/when-do-the-clocks-change) the clocks:
go forward 1 hour at 1am on the last Sunday in March,
go back 1 hour at 2am on the last Sunday in October.
In 2019 this civil local time change happens on March 31st and October 27th, but the days slightly change every year. Is there a clean way to know these dates for each input year?
I need to check these "changing time" dates in an automatic way, is there a way to avoid a for loop to check the details of each date to see if it is a "changing time" date?
At the moment I am exploring these dates for 2019 just to try to figure out a reproducible/automatic procedure and I found this:
# using datetime from the standard library
march_utc_30 = datetime.datetime(2019, 3, 30, 0, 0, 0, 0, tzinfo=datetime.timezone.utc)
march_utc_31 = datetime.datetime(2019, 3, 31, 0, 0, 0, 0, tzinfo=datetime.timezone.utc)
april_utc_1 = datetime.datetime(2019, 4, 1, 0, 0, 0, 0, tzinfo=datetime.timezone.utc)
# using pandas timestamps
pd_march_utc_30 = pd.Timestamp(march_utc_30) #, tz='UTC')
pd_march_utc_31 = pd.Timestamp(march_utc_31) #, tz='UTC')
pd_april_utc_1 = pd.Timestamp(april_utc_1) #, tz='UTC')
# using pandas wrappers
pd_local_march_utc_30 = pd_march_utc_30.tz_convert('Europe/London')
pd_local_march_utc_31 = pd_march_utc_31.tz_convert('Europe/London')
pd_local_april_utc_1 = pd_april_utc_1.tz_convert('Europe/London')
# then printing all these dates
print("march_utc_30 {} pd_march_utc_30 {} pd_local_march_utc_30 {}".format(march_utc_30, pd_march_utc_30, pd_local_march_utc_30))
print("march_utc_31 {} pd_march_utc_31 {} pd_local_march_utc_31 {}".format(march_utc_31, pd_march_utc_31, pd_local_march_utc_31))
print("april_utc_1 {} pd_april_utc_1 {} pd_local_april_utc_1 {}".format(april_utc_1, pd_april_utc_1, pd_local_april_utc_1))
The output of those print statements is:
march_utc_30 2019-03-30 00:00:00+00:00 pd_march_utc_30 2019-03-30 00:00:00+00:00 pd_local_march_utc_30 2019-03-30 00:00:00+00:00
march_utc_31 2019-03-31 00:00:00+00:00 pd_march_utc_31 2019-03-31 00:00:00+00:00 pd_local_march_utc_31 2019-03-31 00:00:00+00:00
april_utc_1 2019-04-01 00:00:00+00:00 pd_april_utc_1 2019-04-01 00:00:00+00:00 pd_local_april_utc_1 2019-04-01 01:00:00+01:00
I could use a for loop to find out if the current date is the last Sunday of the month, or compare the "hour delta" between the current date and the date of the day after to see if there is a +1, but I am wondering if there is a cleaner way to do this?
Is there something attached to the year e.g. knowing the input year is 2019 then we know for sure the "change date" in March will be day 31st?

Answer 1

Using dateutil.rrule can help (install with pip install python-dateutil).
Because we can fetch dates by weeks, we don't need any loops,
from dateutil.rrule import rrule, WEEKLY
from dateutil.rrule import SU as Sunday
from datetime import date
import datetime
def get_last_sunday(year, month):
date = datetime.datetime(year=year, month=month, day=1)
# we can find max 5 sundays in a months
days = rrule(freq=WEEKLY, dtstart=date, byweekday=Sunday, count=5)
# Check if last date is same month,
# If not this couple year/month only have 4 Sundays
if days[-1].month == month:
return days[-1]
else:
return days[-2]
def get_march_switch(year):
# Get 5 next Sundays from first March
day = get_last_sunday(year, 3)
return day.replace(hour=1, minute=0, second=0, microsecond=0)
def get_october_switch(year):
day = get_last_sunday(year, 10)
return day.replace(hour=2, minute=0, second=0, microsecond=0)
print('2019:')
print(' {}'.format(get_march_switch(2019)))
print(' {}'.format(get_october_switch(2019)))
print('2021:')
print(' {}'.format(get_march_switch(2021)))
print(' {}'.format(get_october_switch(2021)))
get_sundays() returns the 5 next sundays from the first day of the given month, because a month can have maximum 5 sundays.
Then I just check (within get_(march|october)_switch()) if the last given sunday is from the expected month, if not well this month only have 4 sunday, I took this one.
Finally I fix the hours, seconds and microseconds.
Output:
2019:
2019-03-31 01:00:00
2019-10-27 02:00:00
2021:
2021-03-28 01:00:00
2021-10-24 02:00:00

Answer 2

I know the topic is quite old now. However, I had the same question today, and at the end I found a solution which seems quite simple to me, using only the standard datetime:
I want to check whether my date refdate is the October DST day - I did it in the following way:
refdate is my standard datetime object.
If you have a panda timestamp, you can convert it to native datetime using .to_pydatetime()
if refdate.month == 10 and refdate.weekday() == 6 and (refdate + dt.timedelta(weeks = 1)).month == 11:
oct_dst = 1