Refactor dates being passed around

I believe this way is nicer to read, we don't have to change between
datetimes and strings and back.

app/celery/letters_pdf_tasks.py

@@ -128,15 +128,17 @@ def collate_letter_pdfs_for_day():
     that have not yet been sent.
     If run after midnight, it will collect up letters created before 5:30pm the day before.
     """
-    date = convert_utc_to_bst(datetime.utcnow())
+    print_run_date = convert_utc_to_bst(datetime.utcnow())
-    if date.time() < LETTER_PROCESSING_DEADLINE:
+    if print_run_date.time() < LETTER_PROCESSING_DEADLINE:
-        date = date - timedelta(days=1)
+        print_run_date = print_run_date - timedelta(days=1)
 
-    # Using the truncated date is ok because UTC to BST does not make a difference to the date,
+    # We can truncate the datetime to a date and add our own time (5:30pm) because UTC to BST does not
-    # since it is triggered mid afternoon.
+    # make a difference to the date since it is triggered mid afternoon.
-    print_run_date = date.strftime("%Y-%m-%d")
+    print_run_deadline = print_run_date.replace(
+        hour=17, minute=30, second=0, microsecond=0
+    )
 
-    letters_to_print = get_key_and_size_of_letters_to_be_sent_to_print(print_run_date)
+    letters_to_print = get_key_and_size_of_letters_to_be_sent_to_print(print_run_deadline)
 
     for i, letters in enumerate(group_letters(letters_to_print)):
         filenames = [letter['Key'] for letter in letters]
@@ -144,7 +146,7 @@ def collate_letter_pdfs_for_day():
         hash = urlsafe_b64encode(sha512(''.join(filenames).encode()).digest())[:20].decode()
         # eg NOTIFY.2018-12-31.001.Wjrui5nAvObjPd-3GEL-.ZIP
         dvla_filename = 'NOTIFY.{date}.{num:03}.{hash}.ZIP'.format(
-            date=print_run_date,
+            date=print_run_deadline.strftime("%Y-%m-%d"),
             num=i + 1,
             hash=hash
         )
@@ -167,8 +169,8 @@ def collate_letter_pdfs_for_day():
         )
 
 
-def get_key_and_size_of_letters_to_be_sent_to_print(print_run_date):
+def get_key_and_size_of_letters_to_be_sent_to_print(print_run_deadline):
-    letters_awaiting_sending = dao_get_letters_to_be_printed(print_run_date)
+    letters_awaiting_sending = dao_get_letters_to_be_printed(print_run_deadline)
 
     letter_pdfs = []
     for letter in letters_awaiting_sending:

app/dao/notifications_dao.py

@@ -731,16 +731,12 @@ def notifications_not_yet_sent(should_be_sending_after_seconds, notification_typ
     return notifications
 
 
-def dao_get_letters_to_be_printed(print_run_date):
+def dao_get_letters_to_be_printed(print_run_deadline):
     """
-    Given a date for a print run, Return all letters created before 5:30pm that day that have not yet been sent
+    Return all letters created before the print run deadline that have not yet been sent
     """
-    last_processing_deadline = datetime.strptime(print_run_date, "%Y-%m-%d").replace(
-        hour=17, minute=30, second=0, microsecond=0
-    )
-
     notifications = Notification.query.filter(
-        Notification.created_at < convert_bst_to_utc(last_processing_deadline),
+        Notification.created_at < convert_bst_to_utc(print_run_deadline),
         Notification.notification_type == LETTER_TYPE,
         Notification.status == NOTIFICATION_CREATED,
         Notification.key_type == KEY_TYPE_NORMAL

tests/app/celery/test_letters_pdf_tasks.py

@@ -296,7 +296,7 @@ def test_get_key_and_size_of_letters_to_be_sent_to_print(notify_api, mocker, sam
         {'ContentLength': 3},
     ])
 
-    results = get_key_and_size_of_letters_to_be_sent_to_print('2020-02-17')
+    results = get_key_and_size_of_letters_to_be_sent_to_print(datetime.now() - timedelta(minutes=30))
 
     assert mock_s3.call_count == 3
     mock_s3.assert_has_calls(