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  When you boot a PC/AT or PS/2, the ROM BIOS start-up routines read the
  time of day from the real-time clock and convert it into the corresponding
  number of timer ticks. This value is used to initialize the 4-byte count
  stored at 0040:006CH in the ROM BIOS data area. All versions of DOS use
  this count value to determine the current time of day. Starting in version
  3.0, DOS also obtains the current date from the real-time clock and
  initializes its own internal record of the date at boot-up time.

  To work with the current date and time in a program, we recommend that you
  use the DOS date and time services (Chapter 16) to get and set the
  current values. You could also use ROM BIOS services to access the
  real-time clock (Chapter 10). However, if you call the ROM BIOS to change
  the date or time, DOS may not be aware of the change and may assume an
  incorrect time or date.