Can anybody please tell me how to get the Unix timestamp from a certain amount of days ago?

To get the current Unix timestamp I can do this.

Code:
(Now - DateSerial(1970, 1, 1)) * 86400
But I need the Unix timestamp from let's say 75 days ago. It doesn't have to be precisely on the second, but I need to delete data that is a certain amount of days old by comparing Unix timestamps.