As with zoneinfo, a user of Microsoft Windows configures DST by specifying the name of a location, and the operating system then consults a table of rule sets that must be updated when DST rules change. Procedures for specifying the name and updating the table vary with release. Updates are not issued for older versions of Microsoft Windows. Windows Vista supports at most two start and end rules per time zone setting. In a Canadian location observing DST, a single Vista setting supports both 1987–2006 and post-2006 time stamps, but mishandles some older time stamps. Older Microsoft Windows systems usually store only a single start and end rule for each zone, so that the same Canadian setting reliably supports only post-2006 time stamps.
When does the table of rule sets the Windows operating system uses have to be updated?
when DST rules change