@FirozShaik  based on my analysis, pega calls function pxDifferenceInDays. if the database is Oracle and column types are timestamp. it applies TRUNC 

 <p:when test=".pyParameters(1).pyDBDataType =='TIMESTAMP' && .pyParameters(2).pyDBDataType =='TIMESTAMP'">
          TRUNC({2}) - TRUNC({1})
        </p:when>

I tried some sample data in live oracle as well. it is working fine.

Create table timestamp_test (id number, timestamp_1 timestamp, timestamp_2 timestamp);
Insert into timestamp_test values(1,systimestamp,systimestamp);
select * from timestamp_test;
select trunc(timestamp_1)-trunc(timestamp_2)  as result from timestamp_test;