Standardise how we query midnight-to-midnight

Partially addresses [1] (lots more detail to read in the comment).
I've also added some tests for the status DAO function to confirm
it behaves as expected across timezones.

[1]: https://github.com/alphagov/notifications-api/pull/3437#discussion_r802634913

app/dao/fact_billing_dao.py

@@ -1,7 +1,7 @@
-from datetime import date, datetime, time, timedelta
+from datetime import date, datetime, timedelta
 
 from flask import current_app
-from notifications_utils.timezones import convert_bst_to_utc, convert_utc_to_bst
+from notifications_utils.timezones import convert_utc_to_bst
 from sqlalchemy import Date, Integer, and_, desc, func
 from sqlalchemy.dialects.postgresql import insert
 from sqlalchemy.sql.expression import case, literal
@@ -317,8 +317,8 @@ def delete_billing_data_for_service_for_day(process_day, service_id):
 
 
 def fetch_billing_data_for_day(process_day, service_id=None, check_permissions=False):
-    start_date = convert_bst_to_utc(datetime.combine(process_day, time.min))
-    end_date = convert_bst_to_utc(datetime.combine(process_day + timedelta(days=1), time.min))
+    start_date = get_london_midnight_in_utc(process_day)
+    end_date = get_london_midnight_in_utc(process_day + timedelta(days=1))
     current_app.logger.info("Populate ft_billing for {} to {}".format(start_date, end_date))
     transit_data = []
     if not service_id:

app/dao/fact_notification_status_dao.py

@@ -1,6 +1,5 @@
-from datetime import datetime, time, timedelta
+from datetime import datetime, timedelta
 
-from notifications_utils.timezones import convert_bst_to_utc
 from sqlalchemy import Date, case, func
 from sqlalchemy.dialects.postgresql import insert
 from sqlalchemy.sql.expression import extract, literal
@@ -36,8 +35,8 @@ from app.utils import (
 
 
 def fetch_status_data_for_service_and_day(process_day, service_id, notification_type):
-    start_date = convert_bst_to_utc(datetime.combine(process_day, time.min))
-    end_date = convert_bst_to_utc(datetime.combine(process_day + timedelta(days=1), time.min))
+    start_date = get_london_midnight_in_utc(process_day)
+    end_date = get_london_midnight_in_utc(process_day + timedelta(days=1))
 
     # query notifications or notification_history for the day, depending on their data retention
     service = Service.query.get(service_id)

app/utils.py

@@ -8,7 +8,7 @@ from notifications_utils.template import (
     LetterPrintTemplate,
     SMSMessageTemplate,
 )
-from notifications_utils.timezones import convert_utc_to_bst
+from notifications_utils.timezones import convert_bst_to_utc, convert_utc_to_bst
 from sqlalchemy import func
 
 DATETIME_FORMAT_NO_TIMEZONE = "%Y-%m-%d %H:%M:%S.%f"
@@ -64,9 +64,7 @@ def get_london_midnight_in_utc(date):
      :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 = 2016-06-16 23:00:00
     """
-    return local_timezone.localize(datetime.combine(date, datetime.min.time())).astimezone(
-        pytz.UTC).replace(
-        tzinfo=None)
+    return convert_bst_to_utc(datetime.combine(date, datetime.min.time()))
 
 
 def get_midnight_for_day_before(date):

tests/app/dao/test_fact_notification_status_dao.py

@@ -14,6 +14,7 @@ from app.dao.fact_notification_status_dao import (
     fetch_notification_status_totals_for_all_services,
     fetch_notification_statuses_for_job,
     fetch_stats_for_all_services_by_date_range,
+    fetch_status_data_for_service_and_day,
     get_total_notifications_for_date_range,
 )
 from app.models import (
@@ -607,3 +608,23 @@ def test_get_total_notifications_for_date_range(sample_service):
 
     assert len(results) == 1
     assert results[0] == ("2021-03-01", 15, 20, 3)
+
+
+@pytest.mark.parametrize('created_at_utc,process_day,expected_count', [
+    # Clocks change on the 27th of March 2022, so the query needs to look at the
+    # time range 00:00 - 23:00 (UTC) thereafter.
+    ('2022-03-27T00:30', date(2022, 3, 27), 1),  # 27/03 00:30 GMT
+    ('2022-03-27T22:30', date(2022, 3, 27), 1),  # 27/03 23:30 BST
+    ('2022-03-27T23:30', date(2022, 3, 27), 0),  # 28/03 00:30 BST
+    ('2022-03-26T23:30', date(2022, 3, 26), 1),  # 26/03 23:30 GMT
+])
+def test_fetch_status_data_for_service_and_day_respects_gmt_bst(
+    sample_template,
+    sample_service,
+    created_at_utc,
+    process_day,
+    expected_count,
+):
+    create_notification(template=sample_template, created_at=created_at_utc)
+    rows = fetch_status_data_for_service_and_day(process_day, sample_service.id, SMS_TYPE)
+    assert len(rows) == expected_count