Use calendar.timegm instead of time.mktime for UTC timestamps

[fenuks]

time.mktime() converts a time tuple in local time to seconds since the Epoch, as stated in the docs:

Convert a time tuple in local time to seconds since the Epoch.
Note that mktime(gmtime(0)) will not generally return zero for most
time zones; instead the returned value will either be equal to that
of the timezone or altzone attributes on the time module.

calendar.timegm() is guaranteed to produce UTC timestamp:

Unrelated but handy function to calculate Unix timestamp from GMT.