No, that's what I consider a brute-force solution. That sort of thing would work, though, and yours and Shonky's solutions are viable if we don't have a math solution.

By math solution, I'm talking about something that simply needs a math equation to solve the problem. Something along the lines of "IF (T1-T0) MOD 900" or something like that. Something that doesn't require iterating through a list for each time stamp. I don't know if that's possible.