Hello Venkata,
Have you tried creating a view on top of the database table.
If you have a specific date range then you create a view on top of the database table and then under the filter tab have the following condition:
"SAP connection".Date_Table.Date_Field >= to_date('2016.03.14','yyyy.mm.dd') AND "SAP connection".Date_Table.Date_Field <= to_date('2016.08.01','yyyy.mm.dd').
This would only show that date range.
Then after that you could create a validation rule that is "SAP CFL".Date_Table.Date_Field = to_date('1900.01.01','yyyy.mm.dd') (I believe that is auto generated blank/null date value) or you conversely select NULL or Blank check from the drop down menu's when you first try to create a validation rule.
After that bind the validation rule to the field in the view and run it under the task.
Let me know if that works for you.