You could store in your player savegame the date, when the user lastly claimed his daily reward. You can compare these stored value + 1 with now. If its greater, the user waited at least 24h.
The formula would be something like this:
(PlayerSavegameDate + 1Day) < Now. If that is true, than you can reward your player again and set PlayerSavegameDate to now.
I hope I could point you to the direction! Good luck with your project
You have to make a blueprint which inherits from “Savegame” (or something like this). Than you can add variables (in your case of type Date) like in any other blueprint. You can then load and save this object with the “Load Game From Slot” or “Save Game To Slot” nodes. Look for further information here