Grimrock

Time.deltaTime() apparently gives you the number of seconds since the last frame.

But what's the difference between Time.currentTime() and Time.systemTime()?

Time.systemTime() seems to be elapsed time since the the Grimrock 2 executable was started, carrying over across pretty much everything except closing the game entirely - even starting/exiting the editor. Also, it's not the number of seconds, it's slower than that. You would use this for something like seeding a PRNG.

Time.currentTime() is the number of seconds elapsed in the dungeon. If you save the game with a current time of 50 seconds, then load the game later, Time.currentTime() will still return 50 seconds when the game is reloaded. You would use this for things like noise for flickering light sources.