The PC maintains two system dates one is part of the hardware on the motherboard, and the other is held by the operating system. The hardware system date is held in the CMOS Real Time Clock (RTC). The other is held by the Operating System such as DOS/Windows/NT.

When the PC is turned on the operating system attempts to synchronize its date and time with the RTC. This occurs during the boot process, the operating system reads the date from the RTC, and it attempts to convert the date to the number of days that has elapsed since 01/01/1980. The original PC maintained the CMOS RTC date as a pair of digits representing MM/DD/YY, and the century byte remains at "19".

Since the century byte is always at "19" and it does not change with the time. When the year goes from "1999" to "2000" the RTC will have the date stored as "1900" instead of "2000". If a computer is on during the transition (December 31, 1999 at midnight) the operating system will have the correct time, however, the RTC will have "1900" instead of "2000".

The PC's that exhibit this flaw, demonstrate it when the system is powered off then back on after the year reaches "2000". The operating system will attempt to synchronize its date and time during the boot process by reading the date from the CMOS RTC, and converting it to the number of days-elapsed since 01/01/1980.  However, when the operating system reads the date as "1900" it sees that date as out of range, and sets its date to 01/04/1980. Some applications that get their date from the CMOS RTC rather than the operating system will receive the year as "1900" instead of "2000".

Our diagnostics utilities will set the date to December 31, 1999 at 11:59:56pm, and utilizes the BIOS 1AH functions 02 & 04h Read Real Time Clock Day Time of Day Services. It will verify if the Year and Century Bytes properly progress to the Year 2000. Next they change the date to February 28, 2000 at 11:59:56pm to verify if the leap year dates are handled properly. The reboot test verifies that the date does not change when the unit is either powered off or rebooted. Click Here to go to on-line diagnostics, downloadable utility.

• The BIOS date increments from 1999 to 2000, normally referred to as representing dates in a millennium MM/DD/YYYY format.
• The BIOS handles the Leap Year in the Year 2000 properly.
• The correct BIOS date is maintained after the Year 2000 even when powered off or rebooted. 

Y2KANSWR.COM will check for the year reaching 2000, and changes the century byte in the RTC from "19" to "20" once the year reaches 2000. The century byte is located in two different locations on the PC, which is determined by the exact type of PC. It can be either IBM Compatible or the IBM PS/2 PC the RTC byte locations are "32H" or "37H", respectively. This utility program also intercepts the 1AH function 04h BIOS Read Real Time Clock Day Time of Day Services, and will put the "20" in the correct century byte field one the year reaches 2000. The Terminate and Stay Resident (TSR) program uses less than 500 bytes of memory space to accomplish these required tasks.

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