mktime() like function to produce GMT?

[M.-A. Lemburg]

Mark Nottingham wrote:
> 
> M.-A. Lemburg wrote [referring to mktime_tz()]:
> > This doesn't always work. For one, time.timezone is set at
> > module init time and does not necessarily apply to the UTC
> > offset (depending on the platform). And second, mktime()
> > with DST set to 0 will not return a UTC time value on all
> > platforms: some implementations simply ignore the value
> > and recalculate it depending on the local time broken
> > down values.
> 
> This explanation fits well. After looking around the linux code (and being
> somewhat shocked to learn that (according to them), it isn't POSIXly correct
> to accept a TZ env variable), this appears to be the case (although my grasp
> of linux internals is tenuous at best).
> 
> > I've had much trouble with this in mxDateTime, but finally found
> > a rather well working trick that only relies on mktime()
> > producing correct local time values.
> 
> Is it doable in Python-only (sorry, I've only taken a cursory glance over
> the mxDateTime module; it looks very elegant and functional, but I'd like to
> try and make the application use only standard library functions)?

You can try to use the Python prototype I hacked up to test
the hack (the documentation mentioned in the copyright notice
referrs to the mxDateTime documentation). It will be included
in the next mxDateTime release.

""" A timegm() emulation for platforms that do not provide that C lib
    API.

    This is the prototype I used to code the timegm() C emulation
    in mxDateTime. It offers a little more than is really needed...

    (c) Copyright Marc-Andre Lemburg; All Rights Reserved.
    See the documentation for further information on copyrights,
    or contact the author (mal at lemburg.com).

"""
from time import *

__debug__ = 0

def local_offset(ticks):

    if __debug__:
	print 'local:',localtime(ticks)
	print 'GMT:',gmtime(ticks)
    (localyear,localmonth,localday,
     localhour,localminute,localsecond,
     localwday,localyday,localdst) = localtime(ticks)
    (gmyear,gmmonth,gmday,
     gmhour,gmminute,gmsecond,
     gmwday,gmyday,gmdst) = gmtime(ticks)
    if gmday != localday:
	localdate = localyear * 10000 + localmonth * 100 + localday
	gmdate = gmyear * 10000 + gmmonth * 100 + gmday
	if localdate < gmdate:
	    offset = -86400
	else:
	    offset = 86400
    else:
	offset = 0
    return (offset 
	    + (localhour - gmhour) * 3600
	    + (localminute - gmminute) * 60
	    + (localsecond - gmsecond))

def timegm(year,month,day,hour,minute,second,wday,yday,dst):

    try:
	ticks = mktime(year,month,day,hour,minute,second,wday,yday,-1)
	return ticks + local_offset(ticks)
    except OverflowError:
	# Hmm, we may have stumbled into the "missing" hour during a
	# DST switch...
	ticks = mktime(year,month,day,0,0,0,wday,yday,-1) 
	offset = local_offset(ticks)
	return (ticks + offset
		+ 3600 * hour
		+ 60 * minute
		+ second)

def dst(ticks):

    offset = local_offset(ticks)
    for checkpoint in (-8640000,10000000,-20560000,20560000):
	try:
	    reference = local_offset(ticks + checkpoint)
	except OverflowError:
	    continue
	if reference != offset:
	    break
    if __debug__:
	print 'given:',offset,'reference:',reference,'(checkpoint:',checkpoint,')'
    return offset > reference

def _test():

    t = 920710000
    oops = 0
    while 1:
	x = apply(timegm,gmtime(t))
	if x != t:
	    print 'Ooops:',gmtime(t),'t =',t,'diff =',x-t
	    oops = oops + 1
	isdst = localtime(t)[-1]
	if isdst != -1 and isdst != dst(t):
	    print 'Ooops: t =',t,'dst() =',dst(t),'isdst =',isdst
	    oops = oops + 1
	try:
	    t = t + 10011
	except OverflowError:
	    break
    if not oops:
	print 'Works.'
	return 1
    else:
	print 'Got %i warnings.' % oops
	return 0

if __name__ == '__main__':
    _test()

-- 
Marc-Andre Lemburg
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