Refactor the get_midnight functions to return the date in UTC but for the localised time.

So in June when we are in BST June 16, 00:00 BST => June 15 23:00 UTC

app/utils.py

@@ -1,5 +1,6 @@
 from datetime import datetime, timedelta
 
+import pytz
 from flask import url_for
 from app.models import SMS_TYPE, EMAIL_TYPE
 from notifications_utils.template import SMSMessageTemplate, PlainTextEmailTemplate
@@ -30,11 +31,18 @@ def get_template_instance(template, values):
     }[template['template_type']](template, values)
 
 
-def get_midnight_for_date(date):
-    midnight = datetime.combine(date, datetime.min.time())
-    return midnight
+def get_london_midnight_in_utc(date):
+    """
+     This function converts date to midnight as BST (British Standard Time) to UTC,
+     the tzinfo is lastly removed from the datetime because the database stores the timestamps without timezone.
+     :param date: the day to calculate the London midnight in UTC for
+     :return: the datetime of London midnight in UTC, for example 2016-06-17 = 2016-06-17 23:00:00
+    """
+    return pytz.timezone('Europe/London').localize(datetime.combine(date, datetime.min.time())).astimezone(
+        pytz.UTC).replace(
+        tzinfo=None)
 
 
 def get_midnight_for_day_before(date):
     day_before = date - timedelta(1)
-    return get_midnight_for_date(day_before)
+    return get_london_midnight_in_utc(day_before)