What Calendar Do They Use In Singapore?

This morning, bright and early, I recharged my mobile phone. When I say recharged, I mean money-wise and not battery-wise. Optus usually provides you with 60 days to use your credit. This time they issued my credit to be valid until 31/6/2007. That's only six weeks. And that's not taking into account that the 31st of June doesn't exist.

Does Singapore use some strange calendar whereby the last day in June is the 31st (Optus belongs to Singtel)? Anybody got any clues?