add check

README.rst

@@ -5,6 +5,7 @@ busy with making everything timezone aware
 - (done) changed start-time and fixes to timezone aware datetimes
 - (done) removed unnecessary helper functionality
 - check .seconds on delta. should it be 'total_seconds'?
+- check date shift `day_diff`
 - run tests on opensoar, does it still work?
 - run test with pysoar, same results?
 - how to keep backwards compatible? add flag?