Is there a way to make a method or if there is a package to count days or set an expiration date?

What i am trying to do is when an user finish registration and saves his data on firebase the app takes the current date and count 30 days to set an expiration date.

Example: the app reads December 27th and count 30 days to set the expiration to January 26th 2025 to save the expiration date in the firebase so anytime the app opens it will read the expiration date and check if the user can generate a QR code.

I checked many methods but they always set a manual date in the examples or maybe i and not getting the idea correctly.

Any help will be appreciated, thank you.