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XXI is the Roman Numeral for 21 and would be used on a clock using the 24 hour, or military, time system where 000 (or zero hundred) hours is midnight, 1200 (twelve hundred) hours is 12:00 noon, and 2400 (twenty four hundred) hours is midnight again. This system avoids confusion as to which time of day is being referred to...which could be catastrophic in a military situation.

21 or XXI on the clock would refer to 2100 hours which is 9:00PM by conventional time.

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15y ago

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