The problem is that the tm_year field of "struct tm" is just an "int"
(per POSIX), and thus time_t values > 2^31*60*60*24*365 cannot
be converted. This made mktime(3) fail even if no such large time values
were passed in by user code because the algorithm does a binary search
over the time_t range which fails if a probe value cannot be converted.
To fix this, limit the time_t range to be scanned to 55 bits (which
is a bit on the safe side, but still good until y570855533).
This is more a stopgap fix, the overflow should be checked for
at other places as well (eg localtime(3)), and there are some more
limitations in timezone parsing code.
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