move days_ago to utils and make it tz aware

it's used in a few places - it should definitely know what timezones
are and return datetimes rather than dates, which are hard to work with
in terms of figuring out how tz aware they are.

app/dao/__init__.py

@@ -28,8 +28,3 @@ class DAOClass(object):
         db.session.delete(inst)
         if _commit:
             db.session.commit()
-
-
-def days_ago(number_of_days):
-    from datetime import date, timedelta
-    return date.today() - timedelta(days=number_of_days)

app/dao/jobs_dao.py

@@ -4,15 +4,13 @@ from datetime import datetime, timedelta
 from flask import current_app
 from notifications_utils.statsd_decorators import statsd
 from sqlalchemy import (
-    Date as sql_date,
     asc,
-    cast,
     desc,
     func,
 )
 
 from app import db
-from app.dao import days_ago
+from app.utils import days_ago
 from app.models import (
     Job,
     JOB_STATUS_PENDING,
@@ -53,7 +51,7 @@ def dao_get_jobs_by_service_id(service_id, limit_days=None, page=1, page_size=50
         Job.original_file_name != current_app.config['ONE_OFF_MESSAGE_FILENAME'],
     ]
     if limit_days is not None:
-        query_filter.append(cast(Job.created_at, sql_date) >= days_ago(limit_days))
+        query_filter.append(Job.created_at >= days_ago(limit_days))
     if statuses is not None and statuses != ['']:
         query_filter.append(
             Job.job_status.in_(statuses)

app/dao/notifications_dao.py

@@ -21,7 +21,7 @@ from sqlalchemy.sql import functions
 from notifications_utils.international_billing_rates import INTERNATIONAL_BILLING_RATES
 
 from app import db, create_uuid
-from app.dao import days_ago
+from app.utils import days_ago
 from app.errors import InvalidRequest
 from app.models import (
     Notification,
@@ -75,7 +75,6 @@ def dao_get_template_usage(service_id, limit_days=None, day=None):
     ).group_by(
         Notification.template_id
     ).subquery()
-
     query = db.session.query(
         Template.id.label('template_id'),
         Template.name,
@@ -256,8 +255,7 @@ def get_notifications_for_service(
     filters = [Notification.service_id == service_id]
 
     if limit_days is not None:
-        days_ago = date.today() - timedelta(days=limit_days)
-        filters.append(func.date(Notification.created_at) >= days_ago)
+        filters.append(Notification.created_at >= days_ago(limit_days))
 
     if older_than is not None:
         older_than_created_at = db.session.query(

app/utils.py

@@ -39,7 +39,7 @@ 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: 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(
@@ -90,3 +90,10 @@ def get_public_notify_type_text(notify_type, plural=False):
         notify_type_text = 'text message'
 
     return '{}{}'.format(notify_type_text, 's' if plural else '')
+
+
+def days_ago(number_of_days):
+    """
+    Returns midnight a number of days ago. Takes care of daylight savings etc.
+    """
+    return get_london_midnight_in_utc(datetime.utcnow() - timedelta(number_of_days))

tests/app/test_utils.py

@@ -1,11 +1,15 @@
 from datetime import datetime
+
 import pytest
+from freezegun import freeze_time
 
 from app.utils import (
     get_london_midnight_in_utc,
     get_midnight_for_day_before,
     convert_utc_to_bst,
-    convert_bst_to_utc)
+    convert_bst_to_utc,
+    days_ago
+)
 
 
 @pytest.mark.parametrize('date, expected_date', [
@@ -44,3 +48,21 @@ def test_convert_bst_to_utc():
     bst_datetime = datetime.strptime(bst, "%Y-%m-%d %H:%M")
     utc = convert_bst_to_utc(bst_datetime)
     assert utc == datetime(2017, 5, 12, 12, 15)
+
+
+@pytest.mark.parametrize('current_time, expected_datetime', [
+    # winter
+    ('2018-01-10 23:59', datetime(2018, 1, 9, 0, 0)),
+    ('2018-01-11 00:00', datetime(2018, 1, 10, 0, 0)),
+
+    # bst switchover at 1am 25th
+    ('2018-03-25 10:00', datetime(2018, 3, 24, 0, 0)),
+    ('2018-03-26 10:00', datetime(2018, 3, 25, 0, 0)),
+    ('2018-03-27 10:00', datetime(2018, 3, 25, 23, 0)),
+
+    # summer
+    ('2018-06-05 10:00', datetime(2018, 6, 3, 23, 0))
+])
+def test_days_ago(current_time, expected_datetime):
+    with freeze_time(current_time):
+        assert days_ago(1) == expected_datetime