rename days_ago to midnight_n_days_ago

also add some more timezone boundary tests and minor code cleanup

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.utils import days_ago
+from app.utils import midnight_n_days_ago
 from app.errors import InvalidRequest
 from app.models import (
     Notification,
@@ -248,7 +248,7 @@ def get_notifications_for_service(
     filters = [Notification.service_id == service_id]
 
     if limit_days is not None:
-        filters.append(Notification.created_at >= days_ago(limit_days))
+        filters.append(Notification.created_at >= midnight_n_days_ago(limit_days))
 
     if older_than is not None:
         older_than_created_at = db.session.query(