make util.Date and util.Time lt robust with other types

cassandra/util.py

@@ -973,6 +973,8 @@ class Time(object):
                           microsecond=self.nanosecond // Time.MICRO) == other
 
     def __lt__(self, other):
+        if not isinstance(other, Time):
+            return NotImplemented
         return self.nanosecond_time < other.nanosecond_time
 
     def __repr__(self):
@@ -1061,6 +1063,8 @@ class Date(object):
             return False
 
     def __lt__(self, other):
+        if not isinstance(other, Date):
+            return NotImplemented
         return self.days_from_epoch < other.days_from_epoch
 
     def __repr__(self):