python's time module seems a little haphazard. For example, here is a list of methods in there, from the docstring:

time() -- return current time in seconds since the Epoch as a float
clock() -- return CPU time since process start as a float
sleep() -- delay for a number of seconds given as a float
gmtime() -- convert seconds since Epoch to UTC tuple
localtime() -- convert seconds since Epoch to local time tuple
asctime() -- convert time tuple to string
ctime() -- convert time in seconds to string
mktime() -- convert local time tuple to seconds since Epoch
strftime() -- convert time tuple to string according to format specification
strptime() -- parse string to time tuple according to format specification
tzset() -- change the local timezone

Looking at localtime() and its inverse mktime(), why is there no inverse for gmtime() ?

Bonus questions: what would you name the method ? How would you implement it ?

4 Answers

Tom On Best Solutions

There is actually an inverse function, but for some bizarre reason, it's in the calendar module: calendar.timegm(). I listed the functions in this answer.

Mark Roddy On

I always thought the time and datetime modules were a little incoherent. Anyways, here's the inverse of mktime

import time
def mkgmtime(t):
    """Convert UTC tuple to seconds since Epoch"""
    return time.mktime(t)-time.timezone
Chris Jester-Young On

I'm only a newbie to Python, but here's my approach.

def mkgmtime(fields):
    now = int(time.time())
    gmt = list(time.gmtime(now))
    gmt[8] = time.localtime(now).tm_isdst
    disp = now - time.mktime(tuple(gmt))
    return disp + time.mktime(fields)

There, my proposed name for the function too. :-) It's important to recalculate disp every time, in case the daylight-savings value changes or the like. (The conversion back to tuple is required for Jython. CPython doesn't seem to require it.)

This is super ick, because time.gmtime sets the DST flag to false, always. I hate the code, though. There's got to be a better way to do it. And there are probably some corner cases that I haven't got, yet.

Ilialuk On

mktime documentation is a bit misleading here, there is no meaning saying it's calculated as a local time, rather it's calculating the seconds from Epoch according to the supplied tuple - regardless of your computer locality.

If you do want to do a conversion from a utc_tuple to local time you can do the following:

>>> time.ctime(time.time())
'Fri Sep 13 12:40:08 2013'

>>> utc_tuple = time.gmtime()
>>> time.ctime(time.mktime(utc_tuple))
'Fri Sep 13 10:40:11 2013'

>>> time.ctime(time.mktime(utc_tuple) - time.timezone)
'Fri Sep 13 12:40:11 2013'

Perhaps a more accurate question would be how to convert a utc_tuple to a local_tuple. I would call it gm_tuple_to_local_tuple (I prefer long and descriptive names):

>>> time.localtime(time.mktime(utc_tuple) - time.timezone)
time.struct_time(tm_year=2013, tm_mon=9, tm_mday=13, tm_hour=12, tm_min=40, tm_sec=11, tm_wday=4, tm_yday=256, tm_isdst=1)


>>> time.ctime(time.mktime(time.localtime(time.mktime(utc_tuple) - time.timezone)))
'Fri Sep 13 12:40:11 2013'    

Hope this helps, ilia.