A calendar looks like one of the simplest interfaces on the web. Seven columns. A few rows. Some numbers. Maybe a previous and next button. That was roughly how I thought about calendars too. Then I started looking more closely at how calendar interfaces actually work, and the "simple grid" turned into a surprisingly interesting engineering problem. Dates are not always timestamps. Weeks don't start on the same day everywhere. Months aren't the same length. Printing has comp...