I am very surprised by this. I just verified that this gives the expected behavior against SQL Server. In both cases we are generating the standard JDBC syntax for this {fn timestampdiff(...)}

reference: http://docs.oracle.com/javadb/10.8.3.0/ref/rrefjdbc88908.html

I may need to investigate the behavior using the lastest driver from pgjdbc and maybe ask that team for help.