Hello Smart kumar,
If you want to dis-engage the automatic Start /end dates according to Priority,
Refer to the part 1.SPRO: Define Priority of this document: Automatic Task Determination in Notifications . You need to remove the entries of the Priority type assigned to your Notification type from this screen.
(Here the Priority type is 'AC'. You need to remove entries for your Priority type).
KJogeswaraRao