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(