2100 hours is a time in military time notation, which is equivalent to 9:00 PM in standard 12-hour clock notation. When speaking in military time, 2100 is pronounced as "twenty-one hundred hours". To convert military time to standard time, you can subtract 1200 from the military time if it is greater than or equal to 1300, add a colon between the hours and minutes, and add "PM" to the end of the time. Therefore, 2100 military time can be converted to 9:00 PM by subtracting 1200 from 2100, which gives 900, adding a colon between the hours and minutes, and adding "PM" to the end of the time.