Updated Examples and Recipes (markdown)

Examples-and-Recipes.md

@@ -1612,6 +1612,74 @@ Once we have a string of form 2), give this sample code a try (thank you [Aaron]
          cout << format("%F %T %Ez", zt) << ' ' << zt.get_time_zone()->name() << '\n';
       }
 
+***
+
+Update from [Howard](https://github.com/HowardHinnant):
+
+There is now functionality to parse the original format: 
+
+    1) 1999-10-31 01:30:00 US/Pacific PST
+
+And check if the time zone abbreviation is consistent, and in the ambiguous case, use the time zone abbreviation to disambiguate the time stamp:
+
+  #include <iostream>
+  #include <sstream>
+  #include <string>
+  #include "tz.h"
+
+  int main()
+  {
+     using namespace std;
+     using namespace date;
+
+     istringstream inputStream{ "1999-10-31 00:30:00 US/Pacific PST" };
+
+     // Using local_seconds would resolve in ambiguous date exception
+     local_seconds tp;
+     string tz_name;
+     string tz_abbrev;
+     inputStream >> parse("%F %T", tp) >> tz_name >> tz_abbrev;
+
+     // bool operator tells us whether stream was successfully parsed
+     assert(bool(inputStream));
+     
+     // Check for ambiguous and nonexistent timestamps
+     auto zone = locate_zone(tz_name);
+     auto info = zone->get_info(tp);
+     zoned_seconds zt{zone};
+     switch (info.result)
+     {
+     case local_info::unique:
+         zt = tp;  // easy case
+         // One can check that the tz_abbrev is consistent with
+         //   info.first.abbrev if desired.
+         break;
+     case local_info::nonexistent:
+         // time stamp never existed.  Throw an error?
+         // Or here is how map to a unique UTC equivalent:
+         zt = make_zoned(zone, tp, choose::earliest);  // choose::latest also
+                                                       //   gives same answer.
+         break;
+     case local_info::ambiguous:
+         // Use tz_abbrev to break the ambiguity
+         if (info.first.abbrev == tz_abbrev)
+             zt = make_zoned(zone, tp, choose::earliest);
+         else if (info.second.abbrev == tz_abbrev)
+             zt = make_zoned(zone, tp, choose::latest);
+         else
+             throw std::runtime_error(tz_abbrev +
+                  " is not a valid abbreviation for " + tz_name);
+         break;
+     }
+
+     // This will output America/Los_Angeles, because US/Pacific is an alias of it.
+     cout << format("%F %T %Ez", zt) << ' ' << zt.get_time_zone()->name() << '\n';
+  }
+
+This involves getting the `local_info` structure from the `time_zone` for that `local_time`.  The `local_info` will have all of the information about that `time_zone/local_time` combination, including whether there is a unique mapping to UTC, a non-existing mapping (as in the gap created by "spring forward"), or an ambiguous mapping (created by a local time occurring twice during a "fall back").
+
+In the ambiguous case you can view both mappings, including their abbreviations, and compare that to the abbreviation you parsed, and then choose either the earlier mapping, or the later mapping.
+
 <a name="microfortnights"></a>
 ### `microfortnights`?!  Are you serious?
 (by [Howard Hinnant](https://github.com/HowardHinnant))