Computer BIOS and Windows NT Daylight Savings Time Settings

The information in this article applies to:

• Microsoft Windows NT Workstation versions 3.5 and 3.51
• Microsoft Windows NT Server versions 3.5 and 3.51

SUMMARY

Some system BIOS adjust the computers built-in real-time clock to compensate for daylight savings time automatically. Windows NT can also be configured to adjust for daylight savings time. If both the BIOS and Windows NT adjust the close for daylight savings time, your clock will be one hour off.

MORE INFORMATION

Windows NT updates its internal date and time every hour by reading the CMOS on the motherboard. In-between the one hour intervals, Windows NT increments its own clock approximately every 10 milliseconds. Because Windows NT cannot detect when the system BIOS/CMOS (hardware level) adjusts the clock for daylight savings time, it may change the clock by one hour, just as the BIOS does. This causes your computer clock to be one hour ahead in the Spring or one hour behind in the Fall after both your computer's BIOS and Windows NT adjust the clock for daylight savings time.

If you encounter this problem, change your BIOS or Windows NT configuration so that only one adjusts for daylight savings.

NOTE: If you boot you computer under both Windows 95 and Windows NT, both operating systems will adjust for daylight savings time independently, also causing the clock to be off by an hour.

For more information on Windows NT daylight savings time implementation, please see the following article(s) in the Microsoft Knowledge Base:

   ARTICLE-ID: Q129574
   TITLE     : Time Stamp Changes with Daylight Savings