Convert an offset in seconds to Datetime in Apex

We often come across a challenge where we want to calculate a future (or a past) Date from the current Date based on an offset that are in seconds.

For e.g., in an OAuth flow once we receive an access_token from an authorization_code grant type,  we also get an expiry time in seconds - this is basically the offset/duration after which the access_token will expire. When the access_token expires, the OAuth flow gives an option to refresh the token using refresh_token flow.

Therefore we need to know when the access_token will expire - not in seconds but the Datetime relative to the Datetime when the access_token was generated.

It isn't a very complex calculation but a bit complex for formula field. Here is a sample apex code to calculate the future Datetime offset from current Datetime:

Apex code to convert seconds to Datetime