Potion duration is bugged.
EG This splash potion of night vision (in the screenshot) is only giving me around 5:15 of night vision when it should be 6:00.

Doesn't work correctly in survival or creative.

EDIT:
Seems to only happen to splash potions; normal potions work correctly.
It does not only occur to Night Vision potions.
My Minecraft is not modded.
Confirmed to happen in 1.6.2 too.