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  Function 2CH (decimal 44) reports the time of day. The time is calculated
  from the ROM BIOS timer-tick count. (See page 59.) DOS responds to the
  ROM BIOS's midnight-passed signal and updates the date every 24 hours.

  The timer-tick count is converted into a meaningful time and placed in
  registers CX and DX. CH contains the hour (0 through 23, on a 24-hour
  clock); CL contains the minutes (0 through 59); DH contains the seconds (0
  through 59); and DL contains hundredths of seconds (0 through 99). This
  function returns AL = 00H if the time is successfully updated, or AL = FFH
  if you specified an invalid time.

  The IBM PC timer ticks 18.2 times per second, so the time of day reported
  by DOS is only as accurate as the timer tick--roughly 5.4 hundredths of a
  second. Nevertheless, even with this relatively low accuracy, you can use
  DOS function 2CH to measure time intervals in many applications.