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

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.