There is actually a link in the Russian wiki page Dave linked to above. The link is on the word time, and it lists the syntax for the time format that can be used for expires.

http://translate.googleusercontent.com/translate_c?hl=en&ie=UTF-8&sl=ru&tl=en&u=http://sysoev.ru/nginx/docs/syntax.html&prev=_t&rurl=translate.google.com&twu=1&usg=ALkJrhiPIr-K56soM05_MMx-BlABaoJkxA

The English wiki needs this added.

All the best,

Todd