Microsoft Windows-based applications that use the TZ environment variable may not work as expected because of changes to daylight saving time (DST). Starting in the spring of 2007, the DST start date and the DST end date for the United States will transition to comply with the Energy Policy Act of 2005. DST in the United States will start three weeks earlier (2:00 A.M. on the second Sunday in March) and will end one week later (2:00 A.M. on the first Sunday in November) than in previous years.
In Windows Vista, in Microsoft Windows Server 2003, and in Microsoft Windows XP, when you use the TZ environment variable, the C Run-Time (CRT) localtime function and the CRT _localtime64 function will not correctly recognize DST. These functions will recognize DST as starting on the first Sunday of April and as ending on the last Sunday of October for all dates.
Note If you have an application that uses the Microsoft C runtime (CRT) library, an update may be required. The recent updates for the CRT do not represent a newly discovered effect on products that were already listed in previous product-specific statements from Microsoft. However, the CRT updates are available for developers to update their applications.
If the CRT behavior affects an application, you can help resolve this behavior by installing the updated CRT library.
The CRT update and the Windows operating system update differ in the following ways:
In Windows Vista, in Microsoft Windows Server 2003, and in Microsoft Windows XP, when you use the TZ environment variable, the C Run-Time (CRT) localtime function and the CRT _localtime64 function will not correctly recognize DST. These functions will recognize DST as starting on the first Sunday of April and as ending on the last Sunday of October for all dates.
Note If you have an application that uses the Microsoft C runtime (CRT) library, an update may be required. The recent updates for the CRT do not represent a newly discovered effect on products that were already listed in previous product-specific statements from Microsoft. However, the CRT updates are available for developers to update their applications.
If the CRT behavior affects an application, you can help resolve this behavior by installing the updated CRT library.
The CRT update and the Windows operating system update differ in the following ways:
- The Windows operating system update enables Windows to recognize DST changes correctly by using the new DST rules for 2007. Microsoft generally recommends this update for every user.
- When applications call CRT libraries after you install the CRT update, the CRT libraries use DST 2007 rules to handle dates correctly. The CRT itself is not an application. A user does not directly interact with the CRT. Instead, the CRT is a DLL that custom applications use.
