If you are in play or run mode, you probably have a script somewhere that’s changing the time of day, however if you are in edit mode, you might have a plugin that’s doing this? but it’s very unlikely that a plugin would be doing this, just look for any script that’s changing the time possibly in Server Script Service or Starter Player or Starter Gui or Starter Pack etc.
Note:
There is no feature or built in way to change the time, only scripts or the properties window can change the time.