I do a lot of UTC conversions, more than most, and I am dismayed that after perhaps a year of relatively frequent mental UTC conversions it's still no faster than firing up a terminal and typing
date -u. Even with Quicksilver and fleet fingers my mind should win that race every time.
No one step is difficult, but there are too many steps. First you have to load in the reference time (might be a UTC time or local time) then maybe convert it to 24-hour time then remember whether DST is in effect or not and whether that means -7 or -6, then figure out whether to add or subtract that 7 or 6 hours, then do the actual subtraction, then wonder if you did that right and possibly wonder whether that's the same date as today or not… Clearly the brute force method is not the way to go.
If Richard Feynman can calculate square roots in his head instantly, I'm sure I can convert time zones. Anyone have any suggestions on how to gain this skill? I think I'll add Mental Math for Pilots to my wishlist.