Time Math is the adding and subtracting of increments of time. Also, figuring out the local time across time zones. Time Math can be extremely difficult during travel.
Time math is much more difficult than regular math in that One Hundred equals Sixty (minutes). If it's 5:49 and you add 22 minutes, what do you got?... See? It's tough.
Say you're flying from Los Angeles to Chicago and need to be at a meeting at 4:45. You need to conduct time math to figure out how long it'll take to get to the airport, what time the flight leaves YOUR time, what the local time will be when you land, how long it'll be to catch a cab, and how long you have to get to the meeting destination.
Another example would be if someone says you're meeting at 2:00 but then calls back to say 'Actually, be there an hour and a quarter before then.'
Time Math can be a bitch when doing 15 minute increments.