The term 'calendar units' refers to the various divisions of time used to measure and organize events. The most common calendar units are MOS (3 letters), which stands for months of the year; DATES (5 letters), which refer to specific days of the month; MONTHS (6 letters), which are the 12 divisions of the year; DAYS (4 letters), which are the 24-hour periods that make up a week; YEARS (5 letters), which are the 12-month cycles that make up a decade; YEAR (4 letters), which is the same as a calendar year; and WEEKS (5 letters), which are the seven-day periods that make up a month.