Differences can exist in: The calendar that is used. The order in which the year, month, and day are represented. (Year-month-day, day-month-year, and month-day-year are the common combinations.) How weeks are identified (see seven-day week) Whether written months are identified by name, by number (1–12), or by Roman numeral (I-XII). Whether the 24-hour clock, 12-hour clock, or 6-hour clock is use