Although the "subtracting hours" method is highly risky, for a variety of reasons, it does work if you use it within reason.

It would be highly advisable to subtract 26 hours from noon on the day in question, which allows for those wacky time zones which shift 1.5 hours. This should put you at anywhere from 10am to 12pm on the day prior, depending on how wacky the time zone in question is. It should never, though put you more than a single day back. The UNIX time system thankfully can't represent time around the Gregorian Reformation of 1582 which skipped 10 days(!) to make the calendar Leap Year Compliant.

ObUselessFact:
BTW, there are more than 60 seconds in a minute. Every year the atomic clock people add or subtract several "leap seconds" to make sure that noon is noon. This is the fundamental difference between GMT and UCT (Universal Coördinated Time). With all the water, air, wind, and tectonic movement, the Earth is not spinning at a precise rate, and corrections must be made to keep things exactly on time. However, unless your computer had an atomic clock in it that was constantly synchronized to UCT, you shouldn't worry too much about this.