This sounds to me very much like a XY Problem. Your first question was (rephrased) how to convert some numbers to a timestamp. Now your question seems to be how to subtract the number you got that way from another string. Maybe you can help us improve the answers you get by telling us exactly what you start out with, what you want to have in the end and what you have tried so far?

Maybe you have two strings that seem to represent timestamps and you want to know the number of seconds between them? Then a convenient solution would be to extract the numbers from these strings and to calculate the epoch times from them, calculate the difference of these epoch times, and then to create a new timestamp-looking-string from those.

But it's hard for us to guess, even though now we have Acme::ESP.