For making a global leaderboard
I made a tutorial on how to make one: How to make a simple global leaderboard
For reseting a global leaderboard daily
The key is just that; Change the key you save to & load from.
For example, the key could be day:month:year. To get the countdown, you do a bit of math to figure out when the clock will reset.
This was a bit brief, so if you have any more questions feel free to ask! 