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  Function 2BH (decimal 43) sets DOS's record of the current date, using the
  same registers as function 2AH. The date is set in CX and DX. DH contains
  the month number (1 through 12); DL contains the day of the month (1
  through 28, 29, 30, or 31, as appropriate); CX contains the year (1980
  through 2099). This function returns AL = 00H if the date is successfully
  updated, or AL = FFH if you specified an invalid date.

  Starting in DOS version 3.3, this function also updates the real-time
  clock/calendar 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 date.