Please Help Me!

Ok I really need to know how to do this very quickly so if anyone can tell me how or give me any links I will be very great full!
Ok what I need to know is im making a credits system so if they had 1 credit and they pressed this button how would I make it
give them donator for like 30 minutes or somthing like that.
So how would I give them a rank for a certain amount of time?

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With a Timestamp, and some Variables