diff -r 760f222103c7 Lib/datetime.py
--- a/Lib/datetime.py	Mon Mar 02 09:36:48 2015 -0500
+++ b/Lib/datetime.py	Tue Mar 03 15:43:23 2015 +0100
@@ -284,7 +284,7 @@ def _check_time_fields(hour, minute, sec
     if not 0 <= minute <= 59:
         raise ValueError('minute must be in 0..59', minute)
     if not 0 <= second <= 59:
-        raise ValueError('second must be in 0..59', second)
+        raise ValueError('second must be in 0..60', second)
     if not 0 <= microsecond <= 999999:
         raise ValueError('microsecond must be in 0..999999', microsecond)
     return hour, minute, second, microsecond
@@ -1322,6 +1322,9 @@ class datetime(date):
             self.__setstate(year, month)
             self._hashcode = -1
             return self
+        if second == 60:
+            # Issue #23574: leap seconds are ignored
+            second = 59
         year, month, day = _check_date_fields(year, month, day)
         hour, minute, second, microsecond = _check_time_fields(
             hour, minute, second, microsecond)
@@ -1385,7 +1388,6 @@ class datetime(date):
             t += 1
             us = 0
         y, m, d, hh, mm, ss, weekday, jday, dst = converter(t)
-        ss = min(ss, 59)    # clamp out leap seconds if the platform has them
         result = cls(y, m, d, hh, mm, ss, us, tz)
         if tz is not None:
             result = tz.fromutc(result)
diff -r 760f222103c7 Lib/test/datetimetester.py
--- a/Lib/test/datetimetester.py	Mon Mar 02 09:36:48 2015 -0500
+++ b/Lib/test/datetimetester.py	Tue Mar 03 15:43:23 2015 +0100
@@ -1625,7 +1625,7 @@ class TestDateTime(TestDate):
         self.theclass(2000, 1, 31, 23, 59, 0)    # no exception
         self.theclass(2000, 1, 31, 23, 59, 59)   # no exception
         self.assertRaises(ValueError, self.theclass, 2000, 1, 31, 23, 59, -1)
-        self.assertRaises(ValueError, self.theclass, 2000, 1, 31, 23, 59, 60)
+        self.assertRaises(ValueError, self.theclass, 2000, 1, 31, 23, 59, 61)
         # bad microseconds
         self.theclass(2000, 1, 31, 23, 59, 59, 0)    # no exception
         self.theclass(2000, 1, 31, 23, 59, 59, 999999)   # no exception
@@ -2094,6 +2094,14 @@ class TestDateTime(TestDate):
         self.assertEqual(dt2.newmeth(-7), dt1.year + dt1.month +
                                           dt1.second - 7)
 
+    def test_leap_second(self):
+        # Issue #23574: leap seconds are ignored
+        dt = self.theclass(2012, 6, 30, 23, 59, 60)
+        self.assertEqual(dt.second, 59)
+
+        self.assertRaises(ValueError, self.theclass, 2012, 6, 30, 23, 59, 61)
+
+
 class TestSubclassDateTime(TestDateTime):
     theclass = SubclassDatetime
     # Override tests not designed for subclass
diff -r 760f222103c7 Modules/_datetimemodule.c
--- a/Modules/_datetimemodule.c	Mon Mar 02 09:36:48 2015 -0500
+++ b/Modules/_datetimemodule.c	Tue Mar 03 15:43:23 2015 +0100
@@ -425,7 +425,7 @@ check_time_args(int h, int m, int s, int
     }
     if (s < 0 || s > 59) {
         PyErr_SetString(PyExc_ValueError,
-                        "second must be in 0..59");
+                        "second must be in 0..60");
         return -1;
     }
     if (us < 0 || us > 999999) {
@@ -4023,6 +4023,9 @@ datetime_new(PyTypeObject *type, PyObjec
     if (PyArg_ParseTupleAndKeywords(args, kw, "iii|iiiiO", datetime_kws,
                                     &year, &month, &day, &hour, &minute,
                                     &second, &usecond, &tzinfo)) {
+        /* Issue #23574: leap seconds are ignored */
+        if (second == 60)
+            second = 59;
         if (check_date_args(year, month, day) < 0)
             return NULL;
         if (check_time_args(hour, minute, second, usecond) < 0)
@@ -4058,14 +4061,6 @@ datetime_from_timet_and_us(PyObject *cls
         return PyErr_SetFromErrno(PyExc_OSError);
     }
 
-    /* The platform localtime/gmtime may insert leap seconds,
-     * indicated by tm->tm_sec > 59.  We don't care about them,
-     * except to the extent that passing them on to the datetime
-     * constructor would raise ValueError for a reason that
-     * made no sense to the user.
-     */
-    if (tm->tm_sec > 59)
-        tm->tm_sec = 59;
     return PyObject_CallFunction(cls, "iiiiiiiO",
                                  tm->tm_year + 1900,
                                  tm->tm_mon + 1,