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

interface/wx/datetime.h

@@ -846,6 +846,15 @@ public:
         them as a wxTimeSpan.
     */
     wxTimeSpan Subtract(const wxDateTime& dt) const;
+    /**
+       Returns the difference between this object and @a dt as a wxDateSpan.
+
+       This method allows to find the number of entire years, months, weeks and
+       days between @a dt and this date.
+
+       @since 2.9.5
+    */
+    wxDateSpan DiffAsDateSpan(const wxDateTime& dt) const;
 
     /**
         Adds the given date span to this object.