db2 => select timestampdiff(  1 , char( current timestamp - current timestamp +  1  ) ) from sysibm.sysdummy1

 1 
-----------
           0 

   1  record(s) selected.

db2 => select timestampdiff(  1 , char( null ) ) from sysibm.sysdummy1
SQL0206N  "NULL" is not valid in the context where it is used.  SQLSTATE= 42703 

db2 => select timestampdiff(  1 , null ) from sysibm.sysdummy1
SQL0206N  "NULL" is not valid in the context where it is used.  SQLSTATE= 42703