From the nvram I get the following when searching for something relevant:
time_zone=-08 1 1
But, it's like impossible to figure out what the fields refer to. I'm GMT+1, if that's of any interest.
Extracted from buildroot/build_mipsel/WRT54GS/release/src/router/www/cisco_wrt54g_en/index.asp
"-12 1 0" (GMT-12:00) Kwajalein
"-11 1 0" (GMT-11:00) Midway Island, Samoa
"-10 1 0" (GMT-10:00) Hawaii
"-09 1 1" (GMT-09:00) Alaska
"-08 1 1" (GMT-08:00) Pacific Time (USA & Canada)
"-07 1 0" (GMT-07:00) Arizona
"-07 2 1" (GMT-07:00) Mountain Time (USA & Canada)
"-06 1 0" (GMT-06:00) Mexico
"-06 2 1" (GMT-06:00) Central Time (USA & Canada)
"-05 1 0" (GMT-05:00) Indiana East, Colombia, Panama
"-05 2 1" (GMT-05:00) Eastern Time (USA & Canada)
"-04 1 0" (GMT-04:00) Bolivia, Venezuela
"-04 2 1" (GMT-04:00) Atlantic Time (Canada), Brazil West
"-03.5 1 1" (GMT-03:30) Newfoundland
"-03 1 0" (GMT-03:00) Guyana
"-03 2 1" (GMT-03:00) Brazil East, Greenland
"-02 1 0" (GMT-02:00) Mid-Atlantic
"-01 1 2" (GMT-01:00) Azores
"+00 1 0" (GMT) Gambia, Liberia, Morocco
"+00 2 2" (GMT) England
"+01 1 0" (GMT+01:00) Tunisia
"+01 2 2" (GMT+01:00) France, Germany, Italy
"+02 1 0" (GMT+02:00) South Africa
"+02 2 2" (GMT+02:00) Greece, Ukraine, Romania, Turkey
"+03 1 0" (GMT+03:00) Iraq, Jordan, Kuwait
"+04 1 0" (GMT+04:00) Armenia
"+05 1 0" (GMT+05:00) Pakistan, Russia
"+06 1 0" (GMT+06:00) Bangladesh, Russia
"+07 1 0" (GMT+07:00) Thailand, Russia
"+08 1 0" (GMT+08:00) China, Hong Kong, Australia Western
"+08 2 0" (GMT+08:00) Singapore, Taiwan, Russia
"+09 1 0" (GMT+09:00) Japan, Korea
"+10 1 0" (GMT+10:00) Guam, Russia
"+10 2 4" (GMT+10:00) Australia
"+11 1 0" (GMT+11:00) Solomon Islands
"+12 1 0" (GMT+12:00) Fiji
"+12 2 4" (GMT+12:00) New Zealand
Note : Don't forget to backslash any special characters (like space) when you do you nvram set time_zone=-05 2 1