Medicine for bladder spasms. Advice please.

I have question is it ok to give my cat his bladder spasm medication about an hour and a half early? Supposed to be every 12 hrs

That is something you should ask the vet. Some medicines can be given a little early or late but others can't. There may be even medicines where if you know if you can't dose on time it's better to skip the dose than to give it too early / too late. Only your vet can tell you if you can safely give your cat's medicine early or not.