I found it quite a bear to update Solaris for the new DST. A patch install is required, but for most of my older Solaris boxes like 8 and 9 I also had prerequisite patches that needed to be installed.
The easiest method I found was to install the Recommended patches and then the DST patch. Also, be careful to ensure that the Recommended doesn't affect you current setup. It just seems like too much work for a date correction.
On a side note, I believe that if you have Java installed on the machine that also requires an update to make it DST complaint.