Bug 11474: Remove errors caused by use of given/when statement

Bug 11474: Remove errors caused by use of given/when statement

This patch replaces a given/when statement by an if so we
do not get warnings on compilation when using Perl 5.18 or later.

Note the perldoc for the subroutine was not correct;
code was not testing that paramater equalled the values
but that it contained them. Have amended accordingly

Have documented behaviour in case parameter contains
neither value.

Subroutine does not appear to me used elsewhere in
codebase

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests and QA script, especially t/DateUtils.t.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>