1. 그레고리력 달력
1752년 9월 그레고리력 채택으로 인해 11일이 삭제됨.
2. cal man page on Linux
Assume the switch from Julian to Gregorian Calendar at the date associ‐ ated with the country_code. If not specified, ncal tries to guess the switch date from the local environment or falls back to September 2, 1752. This was when Great Britain and her colonies switched to the Gre‐ gorian Calendar.