Add wxDateTime::DiffAsDateSpan().

This method returns the difference between the dates as wxDateSpan, unlike the
existing Subtract() and overloaded operator-() that return wxTimeSpan.

Closes #14704.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72600 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/datetime/datetimetest.cpp

@@ -1212,6 +1212,7 @@ void DateTimeTestCase::TestTimeArithmetics()
         CPPUNIT_ASSERT_EQUAL( dt, dt1 - span );
         CPPUNIT_ASSERT_EQUAL( dt, dt2 + span );
         CPPUNIT_ASSERT_EQUAL( dt1, dt2 + 2*span );
+        CPPUNIT_ASSERT_EQUAL( span, dt1.DiffAsDateSpan(dt) );
     }
 }