"adjust my clock manually and then uncheck "Set date & time automatically" in the Date & Time preference panel?"
I'm told this is OK as far as your own perception of what the time is on your computer, but not when it comes to talking to other computers when the UTC offset needs to be known to the OS. So if you're doing lots of internet you're likely to encounter problems pretty quickly. Apple's support for this is most unsatisfactory.
The only straight-forward unofficial (at your own risk) fix is here. NOTE the developers comments and system reqs before you use it!:
http://www.mactcp.org.nz/nzdt.html