1. If someone leaves Vancouver at 1 am and arrives in Seattle at 4 am on the same day, and you know the distance they travelled is 240 km, what was their average rate of speed? (Correct answer: 120 km/h, 80 km/h, or 60 km/h, depending on what day it is. UTC storage helps here.)

2. If it's noon in Vancouver, what time is it in London, England? (Correct answer: 7 pm, 8 pm, or 9 pm, depending on what day it is. Knowledge of regional DST schedule differences helps here.)

It's my experience that when the programmers get date math correct, they may still find themselves fielding "bug" reports from enterprise customers who should understand the concept but don't (particularly if their business is billing or rate calculation -- think of short term billing for hourly parking or CPU hours as an example).