“Your phone stores the predefined DST dates in a table in its memory. Changes in DST causes the following problems for Windows phones in the affected time zones if the update is not applied:
*The phone will have outdated DST rules and will not honor the new DST transition time. The clock will read one hour off actual time during the new DST period.
*Calendar items will be displayed one hour off actual time during the new DST period.”

Download it from MS here.

It appears as though this update is only for specific countries, and the US and Canada aren’t listed. To see the list, go to the above link and scroll down to the table called “Changes in the Windows phone DST update”.