23:21 UTC - twenty-three twenty-one UTC

Clock	Time	Spoken
24-hour clock	23:21	twenty-three twenty-one, twenty-one minutes past twenty-three, thirty-nine minutes to midnight, thirty-nine minutes to zero
12-hour clock	11:21 PM	eleven twenty-one pm, twenty-one minutes past eleven pm, thirty-nine minutes to midnight, thirty-nine minutes to twelve am
Military	2321Z	two-three-two-one hours, twenty-three twenty-one hours

AM or A.M. - latin ante meridiem, meaning before midday, PM or P.M. - latin post meridiem, meaning after midday. Letter 'Z' in military time indicates ZULU Time Zone which is equivalent to UTC.