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  Function 2DH (decimal 45) sets the time of day. The time is specified 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); DL contains hundredths of seconds (0 through 99).

  Starting in DOS version 3.3, this function also updates the real-time
  clock in the PC/AT and PS/2. In earlier versions, you must still use ROM
  BIOS interrupt 1AH services to change the real-time clock time.