converting from local to utc timezone

Question

I\'m attempting to craft a function that takes a time object and converts it to UTC time. The code below appears to be off by one hour. When i run noon through the converter,

Answer 1

To convert time in given timezone to UTC time:

from datetime import datetime
import pytz

def convert_to_utc(time, tzname, date=None, is_dst=None):
    tz = pytz.timezone(tzname)
    if date is None: # use date from current local time in tz
        date = datetime.now(tz).date()

    dt = tz.localize(datetime.combine(date, time), is_dst=is_dst)
    return dt.astimezone(pytz.utc).time(), dt.utcoffset().total_seconds()

if is_dst is None it raises an exception for ambiguous local times.

To convert UTC time to local time in given timezone:

def convert_from_utc(time, tzname, date=None):
    tz = pytz.timezone(tzname)
    if date is None: # use date from current time in utc
        date = datetime.utcnow().date()
    dt = datetime.combine(date, time).replace(tzinfo=pytz.utc)
    return tz.normalize(dt.astimezone(tz)).time()

Example:

time = datetime.strptime('12:00:00', "%H:%M:%S").time()
utc_time, offset = convert_to_utc(time, 'America/Chicago')
print utc_time.strftime("%H:%M:%S"), offset # -> 17:00:00 -18000.0

utc_time = datetime.strptime('17:00:00', "%H:%M:%S").time()
time = convert_from_utc(utc_time, 'America/Chicago')
print time.strftime("%H:%M:%S") # -> 12:00:00

In general it is preferable to work with full datetime objects to avoid ambiguity with what is the correct date i.e., pass and return datetime objects.

Answer 2

By using the replace method on the datetime, you're not allowing the time zone to be adjusted for daylight savings time. Try using one of the documented methods from the pytz documentation:

src_dt = src_tz.localize(dt)

Answer 3

Change

src_dt = dt.replace(tzinfo=src_tz)

to

src_dt = src_tz.localize(dt)

Using localize adjusts for Daylight Savings Time, while replace does not. See the section entitled "Localized times and date arithmetic" in the docs.