How to sum a date?

Question

MySQL

Good afternoon.
How to summarize the date correctly?
Tariffs table:
Id, name, price, period Payments
table:
id, user_id, tariff_id, expiration_at, created_at
There are tariffs for a week, a month, half a year...
When a user purchases one of the tariffs, you need to write the expiration_at in expiration_at .
Please write how to implement it correctly. I will be very grateful.
Thank you very much in advance!

Answer 1

https://momentjs.com/