I had some checks performed on a Windows platform using the following snippet:

# valid for 2016
import time
def check():
    t = time.gmtime()
    print(46*86400*365 + 11*86400 + (t.tm_yday-1)*86400 + t.tm_hour*3600 + t.tm_min*60 + t.tm_sec)
    print(time.time())
    print(time.gmtime(0))
check()

This ensures that the time since the epoch is counted excluding leap seconds if the first two lines of output are approximately the same (to nearest second), and finally that the epoch is the Unix epoch.

On Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 07:18:10) [MSC v.1900 32 bit (Intel)] on win32 I see:

1482502941
1482502941.9609053
time.struct_time(tm_year=1970, tm_mon=1, tm_mday=1, tm_hour=0, tm_min=0, tm_sec=0, tm_wday=3, tm_yday=1, tm_isdst=0)

Unless there is major variation among windows versions on how FILETIMEs are calculated and the results of basic system calls, I feel fairly confident now that the calculation of time since the epoch in CPython is the same on Windows as it is in POSIX-compliant platforms.