[ticket/13216] Use gmdate() instead of date() in datetime tests

PHPBB3-13216
Merge pull request #3067 from marc1706/ticket/13216

[ticket/13216] Use gmdate() instead of date() in datetime tests
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/13216] Use gmdate() instead of date() in datetime tests