added note about timezone conversion to DateValidator

fixes #11692

docs/guide/tutorial-core-validators.md

@@ -149,6 +149,9 @@ specified via [[yii\validators\DateValidator::timestampAttribute|timestampAttrib
   [[yii\validators\DateValidator::$timestampAttributeFormat|$timestampAttributeFormat]] and
   [[yii\validators\DateValidator::$timestampAttributeTimeZone|$timestampAttributeTimeZone]].
   
+  Note, that when using `timestampAttribute`, the input value will be converted to a unix timestamp, which by definition is in UTC, so
+  a conversion from the [[yii\validators\DateValidator::timeZone|input time zone]] to UTC will be performed.
+
 - Since version 2.0.4 it is also possible to specify a [[yii\validators\DateValidator::$min|minimum]] or
   [[yii\validators\DateValidator::$max|maximum]] timestamp.
 

framework/validators/DateValidator.php

@@ -123,6 +123,12 @@ class DateValidator extends Validator
      *
      * This can be the same attribute as the one being validated. If this is the case,
      * the original value will be overwritten with the timestamp value after successful validation.
+     *
+     * Note, that when using this property, the input value will be converted to a unix timestamp,
+     * which by definition is in UTC, so a conversion from the [[$timeZone|input time zone]] to UTC
+     * will be performed. When defining [[$timestampAttributeFormat]] you can control the conversion by
+     * setting [[$timestampAttributeTimeZone]] to a different value than `'UTC'`.
+     *
      * @see timestampAttributeFormat
      * @see timestampAttributeTimeZone
      */