Hello Arm,
You can get the current time usingTimer sservice method, please have a look at this link for the code snippet. To create leaderboard on daily basis, you need to create multiple games through App42 API by name like GameName + Day1, GameName + Day2 and so on. Once your records reaching the last day of month, you can delete the game of first day or first week to reset it.
Please have a look at this tutorial and let me know if you need any other help from my side.
Thanks,
Himanshu Sharma