use LLONG_MIN instead of -1 (which is a valid time value corresponding to 1ms before Epoch) for the invalid wxDateTime m_time value

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@44305 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/datetime.h

@@ -564,7 +564,7 @@ public:
     // ------------------------------------------------------------------------
 
         // default ctor does not initialize the object, use Set()!
-    wxDateTime() { m_time = wxLongLong((long)ULONG_MAX, ULONG_MAX); }
+    wxDateTime() { m_time = wxLongLong(LONG_MIN, 0); }
 
         // from time_t: seconds since the Epoch 00:00:00 UTC, Jan 1, 1970)
 #if (!(defined(__VISAGECPP__) && __IBMCPP__ >= 400))