System settings: make date/time format preview consistent

Summary: BUG: 398493

Reviewers: hein, #vdg, davidedmundson

Reviewed By: #vdg, davidedmundson

Subscribers: GB_2, davidedmundson, abetts, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15423