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+(*----------------------------------------------------------------------------
+    Copyright (c) 2015 Inhabited Type LLC.
+    All rights reserved.
+    Redistribution and use in source and binary forms, with or without
+    modification, are permitted provided that the following conditions
+    are met:
+    1. Redistributions of source code must retain the above copyright
+       notice, this list of conditions and the following disclaimer.
+    2. Redistributions in binary form must reproduce the above copyright
+       notice, this list of conditions and the following disclaimer in the
+       documentation and/or other materials provided with the distribution.
+    3. Neither the name of the author nor the names of his contributors
+       may be used to endorse or promote products derived from this software
+       without specific prior written permission.
+    THIS SOFTWARE IS PROVIDED BY THE CONTRIBUTORS ``AS IS'' AND ANY EXPRESS
+    OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+    WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+    DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE FOR
+    ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+    DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+    OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+    HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+    STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+    POSSIBILITY OF SUCH DAMAGE.
+  ----------------------------------------------------------------------------*)
+
+let parse_date_exn s =
+  try
+    Scanf.sscanf s "%3s, %d %s %4d %d:%d:%d %s"
+      (fun _wday mday mon year hour min sec tz ->
+        let months =
+          [
+            ("Jan", 1);
+            ("Feb", 2);
+            ("Mar", 3);
+            ("Apr", 4);
+            ("May", 5);
+            ("Jun", 6);
+            ("Jul", 7);
+            ("Aug", 8);
+            ("Sep", 9);
+            ("Oct", 10);
+            ("Nov", 11);
+            ("Dec", 12);
+          ]
+        in
+        let parse_tz = function
+          | "" | "Z" | "GMT" | "UTC" | "UT" -> 0
+          | "PST" -> -480
+          | "MST" | "PDT" -> -420
+          | "CST" | "MDT" -> -360
+          | "EST" | "CDT" -> -300
+          | "EDT" -> -240
+          | s ->
+              Scanf.sscanf s "%c%02d%_[:]%02d" (fun sign hour min ->
+                  min + (hour * if sign = '-' then -60 else 60))
+        in
+        let mon = List.assoc mon months in
+        let year =
+          if year < 50 then year + 2000
+          else if year < 1000 then year + 1900
+          else year
+        in
+        let date = (year, mon, mday) in
+        let time = ((hour, min, sec), parse_tz tz * 60) in
+        let ptime = Ptime.of_date_time (date, time) in
+        match ptime with
+        | None -> raise (Invalid_argument "Invalid date string")
+        | Some date -> date)
+  with
+  | Scanf.Scan_failure e -> raise (Invalid_argument e)
+  | Not_found -> raise (Invalid_argument "Invalid date string")
+
+let parse_date s = try Some (parse_date_exn s) with Invalid_argument _ -> None