How to convert local time string to UTC?

Question

How do I convert a datetime string in local time to a string in UTC time?

I\'m sure I\'ve done this before, but can\'t find it and SO will hopefull

Answer 1

I have this code in one of my projects:

from datetime import datetime
## datetime.timezone works in newer versions of python
try:
    from datetime import timezone
    utc_tz = timezone.utc
except:
    import pytz
    utc_tz = pytz.utc

def _to_utc_date_string(ts):
    # type (Union[date,datetime]]) -> str
    """coerce datetimes to UTC (assume localtime if nothing is given)"""
    if (isinstance(ts, datetime)):
        try:
            ## in python 3.6 and higher, ts.astimezone() will assume a
            ## naive timestamp is localtime (and so do we)
            ts = ts.astimezone(utc_tz)
        except:
            ## in python 2.7 and 3.5, ts.astimezone() will fail on
            ## naive timestamps, but we'd like to assume they are
            ## localtime
            import tzlocal
            ts = tzlocal.get_localzone().localize(ts).astimezone(utc_tz)
    return ts.strftime("%Y%m%dT%H%M%SZ")

Answer 2

How about -

time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime(seconds))

if seconds is None then it converts the local time to UTC time else converts the passed in time to UTC.

Answer 3

Here's a summary of common Python time conversions.

Some methods drop fractions of seconds, and are marked with (s). An explicit formula such as ts = (d - epoch) / unit can be used instead (thanks jfs).

• struct_time (UTC) → POSIX (s):
  calendar.timegm(struct_time)
• Naïve datetime (local) → POSIX (s):
  calendar.timegm(stz.localize(dt, is_dst=None).utctimetuple())
  (exception during DST transitions, see comment from jfs)
• Naïve datetime (UTC) → POSIX (s):
  calendar.timegm(dt.utctimetuple())
• Aware datetime → POSIX (s):
  calendar.timegm(dt.utctimetuple())
• POSIX → struct_time (UTC, s):
  time.gmtime(t)
  (see comment from jfs)
• Naïve datetime (local) → struct_time (UTC, s):
  stz.localize(dt, is_dst=None).utctimetuple()
  (exception during DST transitions, see comment from jfs)
• Naïve datetime (UTC) → struct_time (UTC, s):
  dt.utctimetuple()
• Aware datetime → struct_time (UTC, s):
  dt.utctimetuple()
• POSIX → Naïve datetime (local):
  datetime.fromtimestamp(t, None)
  (may fail in certain conditions, see comment from jfs below)
• struct_time (UTC) → Naïve datetime (local, s):
  datetime.datetime(struct_time[:6], tzinfo=UTC).astimezone(tz).replace(tzinfo=None)
  (can't represent leap seconds, see comment from jfs)
• Naïve datetime (UTC) → Naïve datetime (local):
  dt.replace(tzinfo=UTC).astimezone(tz).replace(tzinfo=None)
• Aware datetime → Naïve datetime (local):
  dt.astimezone(tz).replace(tzinfo=None)
• POSIX → Naïve datetime (UTC):
  datetime.utcfromtimestamp(t)
• struct_time (UTC) → Naïve datetime (UTC, s):
  datetime.datetime(*struct_time[:6])
  (can't represent leap seconds, see comment from jfs)
• Naïve datetime (local) → Naïve datetime (UTC):
  stz.localize(dt, is_dst=None).astimezone(UTC).replace(tzinfo=None)
  (exception during DST transitions, see comment from jfs)
• Aware datetime → Naïve datetime (UTC):
  dt.astimezone(UTC).replace(tzinfo=None)
• POSIX → Aware datetime:
  datetime.fromtimestamp(t, tz)
  (may fail for non-pytz timezones)
• struct_time (UTC) → Aware datetime (s):
  datetime.datetime(struct_time[:6], tzinfo=UTC).astimezone(tz)
  (can't represent leap seconds, see comment from jfs)
• Naïve datetime (local) → Aware datetime:
  stz.localize(dt, is_dst=None)
  (exception during DST transitions, see comment from jfs)
• Naïve datetime (UTC) → Aware datetime:
  dt.replace(tzinfo=UTC)

Source: taaviburns.ca

Answer 4

If you already have a datetime object my_dt you can change it to UTC with:

datetime.datetime.utcfromtimestamp(my_dt.timestamp())

Answer 5

Thanks @rofly, the full conversion from string to string is as follows:

time.strftime("%Y-%m-%d %H:%M:%S", 
              time.gmtime(time.mktime(time.strptime("2008-09-17 14:04:00", 
                                                    "%Y-%m-%d %H:%M:%S"))))

My summary of the time/calendar functions:

time.strptime
string --> tuple (no timezone applied, so matches string)

time.mktime
local time tuple --> seconds since epoch (always local time)

time.gmtime
seconds since epoch --> tuple in UTC

and

calendar.timegm
tuple in UTC --> seconds since epoch

time.localtime
seconds since epoch --> tuple in local timezone

Answer 6

Simple

I did it like this:

>>> utc_delta = datetime.utcnow()-datetime.now()
>>> utc_time = datetime(2008, 9, 17, 14, 2, 0) + utc_delta
>>> print(utc_time)
2008-09-17 19:01:59.999996

Fancy Implementation

If you want to get fancy, you can turn this into a functor:

class to_utc():
    utc_delta = datetime.utcnow() - datetime.now()

    def __call__(cls, t):
        return t + cls.utc_delta

Result:

>>> utc_converter = to_utc()
>>> print(utc_converter(datetime(2008, 9, 17, 14, 2, 0)))
2008-09-17 19:01:59.999996