If you really want to stress test time handling code, try to get it to accept Feburary 29, 1900 and Feburary 29, 2000. The first doesn't exist but the second does.
But really, with regards to time use whatever library came with you programming language.
Well, you don't know if 31.12.2015 will have a 23:59:60 or no. These seconds are announced just prior to being added, the system couln't possibly know it.
June is already decided. We'll know either way about the December second when the next bulletin C is published, sometime next July: http://hpiers.obspm.fr/iers/bul/bulc/
But really, with regards to time use whatever library came with you programming language.