diff --git a/app.py b/app.py
index b788213..af5af14 100644
--- a/app.py
+++ b/app.py
@@ -164,10 +164,10 @@ def workouts(user_id):
             db.session.add(heart_rate_reading)
 
         if cadence_readings:
-            start_time = min(
-                c['timestamp'] for c in cadence_readings)
-            end_time = max(
-                c['timestamp'] for c in cadence_readings)
+            timestamps = [datetime_converter(
+                c['timestamp']) for c in cadence_readings]
+            start_time = min(timestamps)
+            end_time = max(timestamps)
             duration = end_time - start_time
             duration = duration.total_seconds()
             average_rpm = sum(
@@ -437,6 +437,10 @@ def format_duration(duration):
         return f"{minutes}m"
 
 
+def datetime_converter(datetime_str: str) -> datetime:
+    return datetime.strptime(datetime_str, "%Y-%m-%dT%H:%M:%S%z")
+
+
 if __name__ == '__main__':
     # Bind to PORT if defined, otherwise default to 5000.
     port = int(os.environ.get('PORT', 5000))