minor #13412 [Form] derive view_timezone from model_timezone (xabbuh)

This PR was merged into the master branch.

Discussion
----------

[Form] derive view_timezone from model_timezone

fixes #12898, requires #13410 to be merged first

Commits
-------

2cd8afa derive view_timezone from model_timezone

Author: javiereguiluz

reference/forms/types/time.rst

@@ -165,6 +165,9 @@ based on this date.
 
 .. include:: /reference/forms/types/options/view_timezone.rst.inc
 
+When no `reference_date`_ is set the ``view_timezone`` defaults to the
+configured `model_timezone`_.
+
 .. caution::
 
     When using different values for `model_timezone`_ and ``view_timezone``,