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.
"Of the" or "on the"
XXI = 21
21. X=10 X= 10 (10+10=20) I=1 therefore 21
This may indicate the time of detonation of an explosive device.
XXI is the Roman numeral for 21.
Hespèrion XXI was created in 1974.
Converxencia XXI was created in 2009.
Freak XXI was created in 1998.
XXI is the same thing as 21.
O stands for Oxygen.
12 thirty on a clock
2 hands of a clock.