Fix placeholder date format
bug #12996 [WebProfilerBundle] Fix placeholder date format (mvar)

This PR was merged into the 2.6 branch.

Discussion
----------

[WebProfilerBundle] Fix placeholder date format

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12995
| License       | MIT
| Doc PR        | -

Commits
-------

9a44aa8 Fix placeholder date format