Split out handling of BST queries for reuse

app/utils.py

@@ -2,6 +2,7 @@ from datetime import datetime, timedelta
 
 import pytz
 from flask import url_for
+from sqlalchemy import func
 from notifications_utils.template import SMSMessageTemplate, PlainTextEmailTemplate
 
 
@@ -46,3 +47,19 @@ def get_london_midnight_in_utc(date):
 def get_midnight_for_day_before(date):
     day_before = date - timedelta(1)
     return get_london_midnight_in_utc(day_before)
+
+
+def get_london_month_from_utc_column(column):
+    """
+     Where queries need to count notifications by month it needs to be
+     the month in BST (British Summer Time).
+     The database stores all timestamps as UTC without the timezone.
+      - First set the timezone on created_at to UTC
+      - then convert the timezone to BST (or Europe/London)
+      - lastly truncate the datetime to month with which we can group
+        queries
+    """
+    return func.date_trunc(
+        "month",
+        func.timezone("Europe/London", func.timezone("UTC", column))
+    )