Find timedelta difference with DST hours (Python)

Question

Given two datetimes such as 2020-01-01 00:00:00 and 2020-04-01 00:00:00, I want to get the timedelta between the two dates in terms of number of hours with any addition/subtraction due to daylight saving time. I am not sure how I can proceed.

Answer 1

By default, Python's timedelta gives you wall-time difference for aware datetime objects (those that have a time zone attached) - not absolute time (in a physical sense; with the second as SI unit).

To get a "DST aware" timedelta, first make sure the datetime objects are localized to a certain time zone (that has the DST). Then, you take into account the UTC offset of the two datetime objects; for example like

from datetime import datetime
from dateutil.tz import gettz

t0, t1 = "2020-03-07 00:00:00", "2020-03-09 00:00:00"
# to datetime object
t0, t1 = datetime.fromisoformat(t0), datetime.fromisoformat(t1)
# set appropriate timezone
tzone = gettz("US/Eastern")
t0, t1 = t0.replace(tzinfo=tzone), t1.replace(tzinfo=tzone)
# check if UTC offset changed
utcdelta = t1.utcoffset() - t0.utcoffset()
# now calculate the timedelta
td = t1 - t0 - utcdelta
print(td)
# 1 day, 23:00:00

For further reading, I recommend the Semantics of timezone-aware datetime arithmetic blog post by Paul Ganssle.