When entering /time set or /time add commands into the chat, it updates the server time, but it doesn't update the client immediately, meaning that for 0-19 ticks the sun, moon and stars stay at their position.
This is an issue if for example you enter /time add 1000 multiple times in one second into the chat or especially when you put time add 12000 into a repeating command block: Then the effects of the day (e.g. burning zombies) appear while it's visually night all the time without the moon moving or the other way around.