How do struct tm tm_isdst and tm_gmtoff interact?

[R. Bernstein]

Nicolas Boullis and Debian users have noticed problems in iso9660
time/date recording and running the regression tests, specifically I
suppose testiso9660. He has also generously provided some patches.

However from this there is the lingering question of how tm_gmtoff and
tm_isdst interact. Does tm_gmtoff reflect the tm_isdst value if it is
1 or not? (tm_isdt can be -1 which means we don't know, 0 for not in
use or 1 for in use)

A scan of the Internet doesn't seem to elucidate; neither does
looking at manual pages for localtime, etc.

Thanks.