Bug 11468: Remove given/when from Koha::Dateutils

Bug 11468: Remove given/when from Koha::Dateutils

given and when give warnings due to their experimental
status as of perl 5.18. Replace the construct with
an if/elsif to avoid the keywords

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>
(cherry picked from commit b529ca00e0cac093fc32a1d123cdbcc84582c155)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>