HPCC-12659 Regression test failure in new Std.Date code

Changed regression tests to call new function rather than
deprecated function; fixed bug in two strftime-based C++
functions that failed to properly fill out the time struct
before calling strftime.

Signed-off-by: Dan S. Camper <dan.camper@lexisnexis.com>
The call to mktime() or the Windows equivalent populates the rest of the struct tm information, which includes the weekday.  Without that call, struct tm contains only the inserted values and weekday would always be zero, which is Sunday, and that's what strftime was picking up with the %a or %A token.