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.