One of the interesting things about UNIX is how it gets stuff right, i.e. how they really paid a lot of attention to detail. A case in point is the calendar command, cal. In 1752, much of the British world switched to Gregorian from Julian calendars, which were steadily drifting. To make up for the drift, 11 days were skipped in the transition. When you type cal 9 1752 on a UNIX workstation, you see the correct calendar:
September 1752
Su Mo Tu We Th Fr Sa
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30