diff --git a/plot_transmission_timeline.py b/plot_transmission_timeline.py
index a6fc112..390e9f4 100755
--- a/plot_transmission_timeline.py
+++ b/plot_transmission_timeline.py
@@ -147,7 +147,7 @@ if __name__ == "__main__":
 
     print(transmission_df)
 
-    range_start_time = transmission_df.index.min()
+    range_start_time = str(transmission_df.index.min())
     range_sum_interval = "{}s".format(args.interval)
 
     # create timedelta range with maximum timedelta