I am trying to convert time from America/New York to UTC and then converting it back to the New York time. But I get different results while doing with this with pytz.
I am doing this:
new_date = parser.parse("May 4, 2021")
new_date = new_date.replace(tzinfo=pytz.timezone("America/New_York"))
date = new_date.astimezone(pytz.timezone("UTC"))
Output:
datetime.datetime(2021, 5, 4, 4, 56, tzinfo=<UTC>)
When I try to reconvert it back to the New York time I get this:
date.astimezone(pytz.timezone("America/New_York"))
I get:
datetime.datetime(2021, 5, 4, 0, 56, tzinfo=<DstTzInfo 'America/New_York' EDT-1 day, 20:00:00 DST>)
My Question is why there is 56 minute difference and what can be done to prevent this?
The 56 min difference originates from the fact that the first entry in the database that pytz accesses, refers to LMT (local mean time):
import pytz
t = pytz.timezone("America/New_York")
print(repr(t))
# <DstTzInfo 'America/New_York' LMT-1 day, 19:04:00 STD>
You can read more in P. Ganssle's Fastest Footgun blog post or in Weird timezone issue with pytz here on SO.
tl;dr - Never replace a tzinfo with a timezone object from pytz! Use localize (or astimezone) instead, to adjust the timezone to the year of the datetime object.
...But: since you use dateutil already, why don't you use it here as well:
import dateutil
new_date = dateutil.parser.parse("May 4, 2021")
# you can safely replace with dateutil's tz objects:
new_date = new_date.replace(tzinfo=dateutil.tz.gettz("America/New_York"))
date = new_date.astimezone(dateutil.tz.UTC)
# date
# datetime.datetime(2021, 5, 4, 4, 0, tzinfo=tzutc())
date = date.astimezone(dateutil.tz.gettz("America/New_York"))
# date
# datetime.datetime(2021, 5, 4, 0, 0, tzinfo=tzfile('US/Eastern'))
print(date)
# 2021-05-04 00:00:00-04:00