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

include/wx/datetime.h

@@ -1063,6 +1063,8 @@ public:
     inline wxTimeSpan Subtract(const wxDateTime& dt) const;
     inline wxTimeSpan operator-(const wxDateTime& dt2) const;
 
+    wxDateSpan DiffAsDateSpan(const wxDateTime& dt) const;
+
     // conversion to/from text
     // ------------------------------------------------------------------------