TCP/IP uses 1.1. 1900, UT as its epoch date, and TCP/IP programs express time as the number of seconds past the epoch date.

re: leap seconds: POSIX gives an arithmetic expression for directly computing a time_t value from a given date/time, and the same relationship is assumed by some (usually older) applications. Any programs creating/dissecting time_t values using such a relationship will typically not handle intervals over leap seconds correctly.