cal is a command-line utility on a number of computer operating systems including Unix, Plan 9, Inferno and Unix-like operating systems such as Linux that prints an ASCII calendar of the given month or year.

[2] The Gregorian calendar reform was adopted by the Kingdom of Great Britain, including its possessions in North America (later to become eastern USA and Canada), in September 1752.

That is the month when 11 days were skipped to make up for lack of leap year adjustments.

Date of adoption of the reform differs widely between countries so, for some users, this feature may be a bug.

Special handling of 1752 is known to have appeared as early as the first edition of the Unix Programmer's Manual in 1971.