Feature: #68022 - Added base date attribute to DateViewHelper

See Issue #68022

Description

The DateViewHelper has been improved with an optional attribute named base. The attribute can be used to define a base-date when using a relative time specification for date. If date is a DateTime object, base is ignored.

The possible relative date format specification can be found in: http://www.php.net/manual/en/datetime.formats.relative.php

<f:format.date format="Y" base="{dateObject}">-1 year</f:format.date>

This will result in the output 2016 assuming the dateObject is some date in 2017.