set unit measure for valid timestamp

handler.py

@@ -36,8 +36,8 @@ class Utterance:
         self.role = None
         self.word_count = self.get_num_words()
         self.timestamp = [starttime, endtime]
-        self.unit_measure = None
-        self.aggregate_unit_measure = endtime
+        self.unit_measure = endtime - starttime if starttime and endtime else None
+        self.aggregate_unit_measure = endtime  
         self.num_math_terms = None
         self.math_terms = None