Add date utility for retention queries (#2299)

* chore: add utility function to calculate date based on retention period

* test(date_utils): add test scenarios for `test_get_query_date_based_on_retention_period()`

app/dao/date_util.py

@@ -1,5 +1,5 @@
 import os
-from datetime import date, datetime, time, timedelta
+from datetime import date, datetime, time, timedelta, timezone
 
 import pytz
 from notifications_utils.strftime_codes import no_pad_month
@@ -104,3 +104,10 @@ def utc_midnight_n_days_ago(number_of_days):
     Returns utc midnight a number of days ago.
     """
     return get_midnight(datetime.utcnow() - timedelta(days=number_of_days))
+
+
+def get_query_date_based_on_retention_period(retention_period):
+    """
+    Computes a date to be used when querying for notifications based on retention period
+    """
+    return datetime.combine(datetime.now(timezone.utc) - timedelta(days=retention_period), time.max)

tests/app/dao/test_date_utils.py

@@ -2,13 +2,15 @@
 
 import pytest
 import pytz
+from freezegun import freeze_time
 
 from app.dao.date_util import (
     get_april_fools,
     get_financial_year,
     get_financial_year_for_datetime,
     get_midnight,
     get_month_start_and_end_date_in_utc,
+    get_query_date_based_on_retention_period,
 )
 
 
@@ -128,3 +130,49 @@ class TestMidnightDateTime:
     def test_get_midnight(self, current_time, expected_midnight):
         actual = get_midnight(current_time)
         assert expected_midnight == actual
+
+
+@freeze_time("2024-09-25 12:25:00")
+@pytest.mark.parametrize(
+    "current_time, retention_period, expected_date",
+    [
+        (
+            datetime(2024, 9, 25, 12, 25, 00),
+            7,
+            datetime(2024, 9, 18, 23, 59, 59, 999999),
+        ),
+        (
+            datetime(2024, 9, 20, 0, 0, 0),
+            7,
+            datetime(2024, 9, 13, 23, 59, 59, 999999),
+        ),
+        (
+            datetime(2024, 9, 10, 23, 59, 59),
+            7,
+            datetime(2024, 9, 3, 23, 59, 59, 999999),
+        ),
+        (
+            datetime(2020, 5, 7, 5, 59, 59),
+            3,
+            datetime(2020, 5, 4, 23, 59, 59, 999999),
+        ),
+        (
+            datetime(2017, 8, 8, 12, 33, 6),
+            3,
+            datetime(2017, 8, 5, 23, 59, 59, 999999),
+        ),
+        (
+            datetime(2014, 8, 8, 18, 0, 59),
+            3,
+            datetime(2014, 8, 5, 23, 59, 59, 999999),
+        ),
+        (
+            datetime(2012, 1, 30, 22, 0, 59),
+            3,
+            datetime(2012, 1, 27, 23, 59, 59, 999999),
+        ),
+    ],
+)
+def test_get_query_date_based_on_retention_period(current_time, retention_period, expected_date):
+    with freeze_time(current_time):
+        assert get_query_date_based_on_retention_period(retention_period) == expected_date