Because of leap seconds, Unix time is also only valid for leap-seconds that the program knows about: in effect, only dates set in the past before the program (or system) was last updated to take new leap seconds into account.
If you need to translate between Unix time and UTC (or something geodesic like UT1), you'll be at most one second off. The difference due to leap seconds only accumulates if you need to translate something civil or geodesic (UTC, UT1, Unix time) to TAI.
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Because of leap seconds, Unix time is also only valid for leap-seconds that the program knows about: in effect, only dates set in the past before the program (or system) was last updated to take new leap seconds into account.
If you need to translate between Unix time and UTC (or something geodesic like UT1), you'll be at most one second off. The difference due to leap seconds only accumulates if you need to translate something civil or geodesic (UTC, UT1, Unix time) to TAI.