No subject

Standard says, which is to say, not much.  The C Standard spec for
mktime is terribly ambiguous and you've found one of the ambiguities.
The C Standard says that mktime can return -1 if attempted in the
``spring forward gap'', but it doesn't say how mktime should determine
whether one is in the gap.

Any user program that messes in this area is relying on behavior that
is not guaranteed by any standard.  For what it's worth, the GNU C
library's mktime agrees with the tz mktime on your example, whereas
Solaris 2.5.1 and BSD/OS 3.0 have the other reasonable interpretation
(i.e. both mktime invocations yield 575002800).