diff --git a/graphene/types/datetime.py b/graphene/types/datetime.py
index d45030b9..ca96547f 100644
--- a/graphene/types/datetime.py
+++ b/graphene/types/datetime.py
@@ -34,8 +34,8 @@ class Date(Scalar):
         try:
             if isinstance(value, datetime.date):
                 return value
-
-            return parse_date(value)
+            elif isinstance(value, str):
+                return parse_date(value)
         except ValueError:
             return None
 
@@ -64,8 +64,8 @@ class DateTime(Scalar):
         try:
             if isinstance(value, datetime.datetime):
                 return value
-
-            return parse_datetime(value)
+            elif isinstance(value, str):
+                return parse_datetime(value)
         except ValueError:
             return None
 
@@ -94,7 +94,7 @@ class Time(Scalar):
         try:
             if isinstance(value, datetime.time):
                 return value
-
-            return parse_time(value)
+            elif isinstance(value, str):
+                return parse_time(value)
         except ValueError:
             return None
diff --git a/graphene/types/tests/test_datetime.py b/graphene/types/tests/test_datetime.py
index 98e5e7ab..47841760 100644
--- a/graphene/types/tests/test_datetime.py
+++ b/graphene/types/tests/test_datetime.py
@@ -2,6 +2,7 @@ import datetime
 
 import pytz
 from graphql import GraphQLError
+import pytest
 
 from ..datetime import Date, DateTime, Time
 from ..objecttype import ObjectType
@@ -88,6 +89,15 @@ def test_datetime_query_variable():
     now = datetime.datetime.now().replace(tzinfo=pytz.utc)
     isoformat = now.isoformat()
 
+    # test datetime variable provided as Python datetime
+    result = schema.execute(
+        """query Test($date: DateTime){ datetime(in: $date) }""",
+        variables={"date": now},
+    )
+    assert not result.errors
+    assert result.data == {"datetime": isoformat}
+
+    # test datetime variable in string representation
     result = schema.execute(
         """query Test($date: DateTime){ datetime(in: $date) }""",
         variables={"date": isoformat},
@@ -100,6 +110,14 @@ def test_date_query_variable():
     now = datetime.datetime.now().replace(tzinfo=pytz.utc).date()
     isoformat = now.isoformat()
 
+    # test date variable provided as Python date
+    result = schema.execute(
+        """query Test($date: Date){ date(in: $date) }""", variables={"date": now}
+    )
+    assert not result.errors
+    assert result.data == {"date": isoformat}
+
+    # test date variable in string representation
     result = schema.execute(
         """query Test($date: Date){ date(in: $date) }""", variables={"date": isoformat}
     )
@@ -112,8 +130,46 @@ def test_time_query_variable():
     time = datetime.time(now.hour, now.minute, now.second, now.microsecond, now.tzinfo)
     isoformat = time.isoformat()
 
+    # test time variable provided as Python time
+    result = schema.execute(
+        """query Test($time: Time){ time(at: $time) }""", variables={"time": time}
+    )
+    assert not result.errors
+    assert result.data == {"time": isoformat}
+
+    # test time variable in string representation
     result = schema.execute(
         """query Test($time: Time){ time(at: $time) }""", variables={"time": isoformat}
     )
     assert not result.errors
     assert result.data == {"time": isoformat}
+
+
+@pytest.mark.xfail(reason="creating the error message fails when un-parsable object is not JSON serializable.")
+def test_bad_variables():
+    def _test_bad_variables(type, input):
+        result = schema.execute(
+            """query Test($input: %s){ %s(in: $input) }""" % (type, type.lower()),
+            variables={"input": input}
+        )
+        assert len(result.errors) == 1
+        # when `input` is not JSON serializable formatting the error message in
+        # `graphql.utils.is_valid_value` line 79 fails with a TypeError
+        assert isinstance(result.errors[0], GraphQLError)
+        print(result.errors[0])
+        assert result.data is None
+
+    not_a_date = dict()
+    not_a_date_str = "Some string that's not a date"
+    today = datetime.date.today()
+    now = datetime.datetime.now().replace(tzinfo=pytz.utc)
+    time = datetime.time(now.hour, now.minute, now.second, now.microsecond, now.tzinfo)
+
+    bad_pairs = [
+        ('DateTime', not_a_date), ('DateTime', not_a_date_str), ('DateTime', today), ('DateTime', time),
+        ('Date', not_a_date), ('Date', not_a_date_str), ('Date', now), ('Date', time),
+        ('Time', not_a_date), ('Time', not_a_date_str), ('Time', now), ('Time', today)
+    ]
+
+    for type, input in bad_pairs:
+        _test_bad_variables(type, input)