How do I use os.time to make a coin spawn in real time?

I want to keep players from rejoining games and collecting coins by setting real time cooldowns. I dont know where to start with os.time. My current coin script is below.

Script: https://docs.google.com/document/d/1ovj05vBYUQi-sYfgLZZHLxtmv30zjVYja6ccyn6rMKI/edit?usp=sharing