Open your squid.conf and search for the line that starts with access_log or cache_access_log and looks like:
access_log /var/log/squid/access.log squid
Of course the path to the logfile may vary in your case.
Note the last keyword (in this case squid, but it could be common, combined, squidmime). Then look for the line that starts with logformat <keyword> and change "%ts.%03tu" with "%tl" fro local time or "%tg" fro GMT time. If the line does not exist you should add it. For the above example it should changed from:
logformat squid %ts.%03tu %6tr %>a %Ss/%03Hs %<st %rm %ru %un %Sh/%<A %mt
logformat squid %tl %6tr %>a %Ss/%03Hs %<st %rm %ru %un %Sh/%<A %mt
You can take a look at squid.conf.default that comes with squid and it's well documented to see other available options for customizing your logs.