> The number of seconds in a day is not fixed (due to leap seconds)

POSIX timestamp doesn't count leap seconds. It looks like the datetime 
module is not aware of the leap seconds:

>>> print datetime.datetime(2006, 1, 1) - datetime.datetime(2005, 12, 
31)
1 day, 0:00:00

About my method: I finally prefer datetime/datetime or 
datetime//datetime instead of a toseconds() method. And to convert a 
timestamp to a timestamp: see my patch attached to issue #2736.