diff --git a/psycopg/typecast_datetime.c b/psycopg/typecast_datetime.c
index 6b94175e..6f063082 100644
--- a/psycopg/typecast_datetime.c
+++ b/psycopg/typecast_datetime.c
@@ -220,7 +220,7 @@ typecast_PYTIME_cast(const char *str, Py_ssize_t len, PyObject *curs)
 static PyObject *
 typecast_PYINTERVAL_cast(const char *str, Py_ssize_t len, PyObject *curs)
 {
-    long years = 0, months = 0, days = 0, sec;
+    long years = 0, months = 0, days = 0, lsec;
     double hours = 0.0, minutes = 0.0, seconds = 0.0, hundredths = 0.0;
     double v = 0.0, sign = 1.0, denominator = 1.0;
     int part = 0;
@@ -317,10 +317,10 @@ typecast_PYINTERVAL_cast(const char *str, Py_ssize_t len, PyObject *curs)
     /* calculates days */
     days += years*365 + months*30;
 
-    micro = (seconds - floor(seconds)) * 1000000.0;
-    sec = (long)floor(seconds);
+    lsec = (long)seconds;
+    micro = (seconds - (double)lsec) * 1000000.0;
     return PyObject_CallFunction((PyObject*)PyDateTimeAPI->DeltaType, "lli",
-                                 days, sec, (int)round(micro));
+                                 days, lsec, (int)round(micro));
 }
 
 /* psycopg defaults to using python datetime types */
diff --git a/tests/test_dates.py b/tests/test_dates.py
index 8bb03c79..10009979 100755
--- a/tests/test_dates.py
+++ b/tests/test_dates.py
@@ -334,8 +334,27 @@ class DatetimeTests(ConnectingTestCase, CommonDatetimeTestsMixin):
         self.assertEqual(t, time(0, 0, tzinfo=FixedOffsetTimezone(330)))
 
     def test_large_interval(self):
+        def total_seconds(d):
+            """Return total number of seconds of a timedelta as a float."""
+            return d.days * 24 * 60 * 60 + d.seconds + d.microseconds / 1000000.0
+
         t = self.execute("select '999999:00:00'::interval")
-        self.assertEqual(t.days * 24 + t.seconds / 60.0 / 60, 999999)
+        self.assertEqual(total_seconds(t), 999999 * 60 * 60)
+
+        t = self.execute("select '-999999:00:00'::interval")
+        self.assertEqual(total_seconds(t), -999999 * 60 * 60)
+
+        t = self.execute("select '999999:00:00.1'::interval")
+        self.assertEqual(total_seconds(t), 999999 * 60 * 60 + 0.1)
+
+        t = self.execute("select '999999:00:00.9'::interval")
+        self.assertEqual(total_seconds(t), 999999 * 60 * 60 + 0.9)
+
+        t = self.execute("select '-999999:00:00.1'::interval")
+        self.assertEqual(total_seconds(t), -999999 * 60 * 60 - 0.1)
+
+        t = self.execute("select '-999999:00:00.9'::interval")
+        self.assertEqual(total_seconds(t), -999999 * 60 * 60 - 0.9)
 
 
 # Only run the datetime tests if psycopg was compiled with support.