Windows Operating System Interactions with BIOS and Real Time Clock



This static CD-based web site is representative of the www.microsoft.com/y2k site as of October 15, 1999.

Microsoft Year 2000 Readiness Disclosure & Resource Center

Windows Operating System Interactions with BIOS and Real Time Clock

3. Introduction

This paper addresses the interaction between the computer's Real Time Clock (RTC) and basic input/output system (BIOS) and the following Microsoft Windows operating systems:

Windows NT« 3.51 and Windows NT« 4.0 û referred to here as Windows NT.

Windows« 98 and Windows« 95 û referred to here as Windows 9.x.

Windows« 3.x and MS-DOS« version 5.x or later.

Most RTC rollover issues are remedied by the PC's BIOS. However in older PCs, the BIOS software may not roll over automatically to 2000. In most of those cases, the century register in the RTC will remain at "19" when the date should roll from 1999 to 2000. An easy workaround for Windows users is to reset the date in Control Panel. The machine's BIOS and operating system should then maintain the date properly.

Microsoft Windows« operating systems insulate applications from most, although not all, Year 2000 rollover issues.

When the Windows« NT operating system starts up, it directly accesses the RTC memory and uses its time and date values to set the system time and date. Whenever Windows NT reads dates between 1900 and 1919 in the RTC it adds 100 to the year. In effect, it maps the time period forward to 2000 û 2019. Windows NT does not add 100 to dates between 1920 and 1999.

Logic built into the Windows« 98 operating system will automatically set the date forward at boot time if the BIOS does not roll over the RTC to 2000. The system does not automatically compensate for BIOSes that change valid dates.

Users of Windows 95, Windows 3.x, and MS-DOS version 5.x or later can address most rollover errors by simply resetting the date in Control Panel or by using the DATE command in MS-DOS.

Microsoft recommends that customers carefully evaluate their systems and test their computing environment from end to end to address overall Year 2000 issues.

Many manufacturers have developed products to help users deal with RTC/BIOS issues and the Year 2000. For links to third party suppliers, select "BIOS" in the Year 2000 Tools section at the Microsoft Year 2000 Readiness Disclosure & Resource Center Web site. Microsoft has not tested, and makes no warranties concerning, any third-party products or resources such as those listed above.

A key part of preparing your computing environment for the turn of the century is knowing what your PC's operating system will do, what the RTC/BIOS will do, and what the RTC/BIOS will do as it performs Power On Self Tests (POST) after the rollover. This paper includes easy-to-use tests to help customers determine whether or not their machine's RTC/BIOS combination and operating system are Year 2000 ready. Microsoft recommends that customers install the free updates for operating systems and applications that Microsoft makes available from time to timeùbefore conducting Year 2000 rollover tests.

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This site is being designated as a Year 2000 Readiness Disclosure and the information contained herein is provided pursuant to the terms hereof and the Year 2000 Information and Readiness Disclosure Act.