Use of `date()` in core depends on PHP timezone set to UTC and not changed by third party code (which cannot be guaranteed).
`gmdate()` is functionally equivalent, but is not affected by PHP timezone setting: it's always UTC, which is the exact behavior the core needs.
Props nielsdeblaauw, Rarst.
Fixes#46438. See #44491.
git-svn-id: https://develop.svn.wordpress.org/trunk@45424 602fd350-edb4-49c9-b593-d223f7449a82