diff --git a/src/Datetime.cpp b/src/Datetime.cpp
index b81d397..6188947 100644
--- a/src/Datetime.cpp
+++ b/src/Datetime.cpp
@@ -166,7 +166,8 @@ bool Datetime::parse (
     return true;
   }
 
-  if (parse_formatted (pig, format))
+  bool ambiguous {false};
+  if (parse_formatted (pig, format, ambiguous))
   {
     // Check the values and determine time_t.
     if (validate ())
@@ -176,6 +177,10 @@ bool Datetime::parse (
       return true;
     }
   }
+  else if (ambiguous)
+  {
+    return false;
+  }
 
   // Allow parse_date_time and parse_date_time_ext regardless of
   // Datetime::isoEnabled setting, because these formats are relied upon by
@@ -232,7 +237,7 @@ void Datetime::clear ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool Datetime::parse_formatted (Pig& pig, const std::string& format)
+bool Datetime::parse_formatted (Pig& pig, const std::string& format, bool& ambiguous)
 {
   // Short-circuit on missing format.
   if (format.empty ())
@@ -246,6 +251,8 @@ bool Datetime::parse_formatted (Pig& pig, const std::string& format)
   int hour   {-1};
   int minute {-1};
   int second {-1};
+  int meridiem {-1};
+  bool twelveHourTime {false};
 
   // For parsing, unused.
   int wday   {-1};
@@ -353,6 +360,60 @@ bool Datetime::parse_formatted (Pig& pig, const std::string& format)
       }
       break;
 
+    case 'i':
+      twelveHourTime = true;
+      if (pig.getDigit (hour))
+      {
+        if (hour == 0)
+          pig.getDigit (hour);
+
+        if (hour == 1)
+        {
+          int tens = hour;
+          if (pig.getDigit (hour))
+            hour += 10 * tens;
+        }
+
+        if (hour < 1 || hour > 12)
+        {
+          pig.restoreTo (checkpoint);
+          return false;
+        }
+      }
+      else
+      {
+        pig.restoreTo (checkpoint);
+        return false;
+      }
+      break;
+
+    case 'I':
+      twelveHourTime = true;
+      if (! pig.getDigit2 (hour) || hour < 1 || hour > 12)
+      {
+        pig.restoreTo (checkpoint);
+        return false;
+      }
+      break;
+
+    case 'p':
+      if (compare (pig.peek (2), "AM", false))
+      {
+        meridiem = 0;
+        pig.skipN (2);
+      }
+      else if (compare (pig.peek (2), "PM", false))
+      {
+        meridiem = 1;
+        pig.skipN (2);
+      }
+      else
+      {
+        pig.restoreTo (checkpoint);
+        return false;
+      }
+      break;
+
     case 'n':
       if (pig.getDigit (minute))
       {
@@ -539,6 +600,27 @@ bool Datetime::parse_formatted (Pig& pig, const std::string& format)
   if (minute == -1) minute = 0;
   if (second == -1) second = 0;
 
+  if (twelveHourTime && meridiem == -1)
+  {
+    ambiguous = true;
+    pig.restoreTo (checkpoint);
+    return false;
+  }
+
+  if (meridiem != -1)
+  {
+    if (hour < 1 || hour > 12)
+    {
+      pig.restoreTo (checkpoint);
+      return false;
+    }
+
+    if (meridiem == 0 && hour == 12)
+      hour = 0;
+    else if (meridiem == 1 && hour != 12)
+      hour += 12;
+  }
+
   _year    = year;
   _month   = month;
   _day     = day;
@@ -3394,6 +3476,9 @@ std::string Datetime::toString (const std::string& format) const
     case 'V': formatted << std::setw (2) << std::setfill ('0') << week ();                break;
     case 'h': formatted                                        << hour ();                break;
     case 'H': formatted << std::setw (2) << std::setfill ('0') << hour ();                break;
+    case 'i': formatted << ((hour () % 12) == 0 ? 12 : (hour () % 12));                break;
+    case 'I': formatted << std::setw (2) << std::setfill ('0') << ((hour () % 12) == 0 ? 12 : (hour () % 12));                break;
+    case 'p': formatted << (hour () < 12 ? "AM" : "PM");                break;
     case 'n': formatted                                        << minute ();              break;
     case 'N': formatted << std::setw (2) << std::setfill ('0') << minute ();              break;
     case 's': formatted                                        << second ();              break;
@@ -3641,6 +3726,9 @@ int Datetime::length (const std::string& format)
     case 'V':
     case 'h':
     case 'H':
+    case 'i':
+    case 'I':
+    case 'p':
     case 'n':
     case 'N':
     case 's':
diff --git a/src/Datetime.h b/src/Datetime.h
index 265ae8b..17638bb 100644
--- a/src/Datetime.h
+++ b/src/Datetime.h
@@ -116,7 +116,7 @@ class Datetime
 
 private:
   void clear ();
-  bool parse_formatted     (Pig&, const std::string&);
+  bool parse_formatted     (Pig&, const std::string&, bool& ambiguous);
   bool parse_named         (Pig&);
   bool parse_epoch         (Pig&);
   bool parse_date_time_ext (Pig&);
diff --git a/test/datetime.t.cpp b/test/datetime.t.cpp
index 5a43be7..570907c 100644
--- a/test/datetime.t.cpp
+++ b/test/datetime.t.cpp
@@ -1070,6 +1070,9 @@ int main (int, char**)
     t.is (Datetime::length ("V"), 2,  "length 'V' --> 2");
     t.is (Datetime::length ("h"), 2,  "length 'h' --> 2");
     t.is (Datetime::length ("H"), 2,  "length 'H' --> 2");
+    t.is (Datetime::length ("i"), 2,  "length 'i' --> 2");
+    t.is (Datetime::length ("I"), 2,  "length 'I' --> 2");
+    t.is (Datetime::length ("p"), 2,  "length 'p' --> 2");
     t.is (Datetime::length ("n"), 2,  "length 'n' --> 2");
     t.is (Datetime::length ("N"), 2,  "length 'N' --> 2");
     t.is (Datetime::length ("s"), 2,  "length 's' --> 2");
@@ -1095,6 +1098,9 @@ int main (int, char**)
     t.is (r32.toString ("d"),        "28", "2015-10-28T12:55:01 -> d ->        28");
     t.is (r32.toString ("H"),        "12", "2015-10-28T12:55:01 -> H ->        12");
     t.is (r32.toString ("h"),        "12", "2015-10-28T12:55:01 -> h ->        12");
+    t.is (r32.toString ("I"),        "12", "2015-10-28T12:55:01 -> I ->        12");
+    t.is (r32.toString ("i"),        "12", "2015-10-28T12:55:01 -> i ->        12");
+    t.is (r32.toString ("p"),        "PM", "2015-10-28T12:55:01 -> p ->        PM");
     t.is (r32.toString ("N"),        "55", "2015-10-28T12:55:01 -> N ->        55");
     t.is (r32.toString ("n"),        "55", "2015-10-28T12:55:01 -> n ->        55");
     t.is (r32.toString ("S"),        "00", "2015-10-28T12:55:01 -> S ->        01");
@@ -1109,6 +1115,16 @@ int main (int, char**)
     t.is (r32.toString ("j"),       "301", "2015-10-28T12:55:01 -> j ->       301");
     t.is (r32.toString ("w"),         "3", "2015-10-28T12:55:01 -> w ->         3");
 
+    Datetime r32a ("2015-10-28T00:05:00");
+    t.is (r32a.toString ("I"),       "12", "2015-10-28T00:05:00 -> I ->        12");
+    t.is (r32a.toString ("i"),       "12", "2015-10-28T00:05:00 -> i ->        12");
+    t.is (r32a.toString ("p"),       "AM", "2015-10-28T00:05:00 -> p ->        AM");
+
+    Datetime r32b ("2015-10-28T13:05:00");
+    t.is (r32b.toString ("I"),       "01", "2015-10-28T13:05:00 -> I ->        01");
+    t.is (r32b.toString ("i"),        "1", "2015-10-28T13:05:00 -> i ->         1");
+    t.is (r32b.toString ("p"),       "PM", "2015-10-28T13:05:00 -> p ->        PM");
+
     // Test all parse options.
     Datetime r33 ("2015 10 28 19 28 01", "Y M D H N S");
     t.is(r33.year (),   2015,         "Y works");
@@ -1126,6 +1142,24 @@ int main (int, char**)
     t.is(r34.minute (),    2,         "n works");
     t.is(r34.second (),    1,         "s works");
 
+    Datetime r34a ("2015 10 28 12 00 AM", "Y M D I N p");
+    t.is(r34a.hour (),      0,         "12 AM parses as hour 0");
+    t.is(r34a.minute (),    0,         "12 AM minute works");
+
+    Datetime r34b ("2015 10 28 12 00 PM", "Y M D I N p");
+    t.is(r34b.hour (),     12,         "12 PM parses as hour 12");
+    t.is(r34b.minute (),    0,         "12 PM minute works");
+
+    Datetime r34c ("2015 10 28 1 30 PM", "Y M D i N p");
+    t.is(r34c.hour (),     13,         "1 PM parses as hour 13");
+    t.is(r34c.minute (),   30,         "1 PM minute works");
+
+    Datetime r34d ("2015 10 28 11 45 pm", "Y M D i N p");
+    t.is(r34d.hour (),     23,         "lowercase pm parses as hour 23");
+    t.is(r34d.minute (),   45,         "lowercase pm minute works");
+
+    t.notok (Datetime::valid ("2015_10_28_1_30", "Y_M_D_i_N"), "ambiguous 12-hour time without AM/PM is invalid");
+
     Datetime r35 ("Wednesday October 28 2015", "A B D Y");
     t.is(r35.year (),    2015,        "Y works");
     t.is(r35.month (),     10,        "B works");