Modify the code to avoid making the following calls incorrectly reported
as important findings by the coverity static analyzer:
- memset() of size 0 at the end of an array
- mktime() on a struct tm that has uninitialized tm_yday
Even though mktime() is documented as ignoring the tm_wday field, the
coverity static analyzer complains about passing an uninitialized value.
Set the field to zero to make it happy.
