Use DateTime::Sub() instead of operator-()

Commit this central place use early.

Change-Id: Ib2e40ae7fbe9dc219ba0a7ec4e0f12a82d7b37b1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/153378
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins

include/unotools/calendarwrapper.hxx

@@ -115,7 +115,7 @@ public:
 
     /// set a local (!) Gregorian DateTime
     void                setGregorianDateTime( const DateTime& rDateTime )
-                                    { setLocalDateTime( rDateTime - aEpochStart ); }
+                                    { setLocalDateTime( DateTime::Sub( rDateTime, aEpochStart)); }
 
 };