Comment 4 for bug 597778

The IERS will only insert a leap second at the last second of a calendar quarter (as previously stated, this is usually the last second of June or December. I believe the SQL standard allows for (but doesn't define) 2 leap seconds in this period (2 leap seconds has never happened in one go yet though). Different databases implement this in different ways. Oracle and MSSQL I believe ignore it completely. DB2 uses 24:00:00 instead (probably a bad idea).

My suggestion is to make the rule stricter: Allow 2 leap seconds but only on the last second of a calendar quarter.