> There are not alway 86400s in 1 day. Midnight is not a whole number of multiples of 86400 after Jan 1st 1970.
Isn't that an issue with the format you want to convert UNIX time to ?
I mean, UNIX time is quite clearly and unambiguously defined as the number of seconds that passed since January 1st 1970. How are UTC's leap seconds any relevant?