diff --git a/app/Dockerfile b/app/Dockerfile
index 0e9bf11..23646a7 100644
--- a/app/Dockerfile
+++ b/app/Dockerfile
@@ -1,4 +1,4 @@
-FROM python:3.6
+FROM python:3.7
 
 LABEL maintainer="Stephen Newey <github@s-n.me>"
 
diff --git a/app/octopus_to_influxdb.py b/app/octopus_to_influxdb.py
index cb70026..01298cc 100644
--- a/app/octopus_to_influxdb.py
+++ b/app/octopus_to_influxdb.py
@@ -79,7 +79,7 @@ def store_series(connection, series, metrics, rate_data):
         if agile_data:
             agile_standing_charge = rate_data['agile_standing_charge'] / 48
             agile_unit_rate = agile_rates.get(
-                measurement['interval_end'],
+                maya.parse(measurement['interval_end']).iso8601(),
                 rate_data[rate]  # cludge, use Go rate during DST changeover
             )
             agile_cost = agile_unit_rate * consumption