If you trade options, have you ever noticed your broker’s IV looks wrong on a Friday? For example, just now, when I looked at the number in the Robinhood App, I knew something was definitely wrong. As it turned out, many Apps use calendar days, ignoring the fact that markets are closed on weekends, to calculate options. This "Time Inflation" creates a massive error in your Greeks. The numbers are still wrong on other days, just not as obvious.