Add a url_converter to check the date format.

app/__init__.py

@@ -85,6 +85,7 @@ from app.notify_client.template_statistics_api_client import (
 from app.notify_client.user_api_client import user_api_client
 from app.url_converters import (
     LetterFileExtensionConverter,
+    SimpleDateTypeConverter,
     TemplateTypeConverter,
 )
 from app.utils import format_thousands, get_logo_cdn_domain, id_safe
@@ -221,6 +222,7 @@ def init_app(application):
     application.url_map.converters['uuid'].to_python = lambda self, value: value
     application.url_map.converters['template_type'] = TemplateTypeConverter
     application.url_map.converters['letter_file_extension'] = LetterFileExtensionConverter
+    application.url_map.converters['simple_date'] = SimpleDateTypeConverter
 
 
 def convert_to_boolean(value):

app/main/views/returned_letters.py

@@ -17,7 +17,7 @@ def returned_letter_summary(service_id):
     )
 
 
-@main.route("/services/<uuid:service_id>/returned-letters-csv/<reported_at>", methods=["GET"])
+@main.route("/services/<uuid:service_id>/returned-letters-csv/<simple_date:reported_at>", methods=["GET"])
 @user_has_permissions('view_activity')
 def returned_letters_report(service_id, reported_at):
     returned_letters = service_api_client.get_returned_letters(service_id, reported_at)

app/url_converters.py

@@ -9,3 +9,7 @@ class TemplateTypeConverter(BaseConverter):
 class LetterFileExtensionConverter(BaseConverter):
 
     regex = '(?:pdf|png)'
+
+
+class SimpleDateTypeConverter(BaseConverter):
+    regex = r'([12]\d{3}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01]))'