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>
Merge pull request #6706 from dcamper/hpcc-12659

HPCC-12659 Regression test failure in new Std.Date code

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>