Several places in icaltime.c use mktime() for its side effects (like
figuring out what day of the week a given date was on). But mktime()
is not defined for dates outside time_t range, and libical doesn't
even check its return value.

This causes problems with things like Exchange/Outlook timezones, which
have DTSTARTs of 1601-01-01 (the Windows equivalent of 1970-01-01).
As a result, libical ends up using stack garbage for a "day of week"
value. I don't know if it actually ends up putting the daylight/standard
time shifts on the wrong days or not. I only noticed the problem because
valgrind complains.