Command Block Issues

Hey, so I've been messing around with command blocks recently and I've realized that some key commands that I need to use aren't working for some reason. The commands I need are /time set 18000 and /weather thunder but neither of them work. I also tested other commands like /say and /tp and they all seem to work. Is there some sort of permission that I need to change or something? Also, I'm using singleplayer if that affects anything.