REST API: Allow to filter orders by datetime

doc/api/resources/orders.rst

@@ -385,6 +385,7 @@ List of all orders
    :query datetime modified_since: Only return orders that have changed since the given date. Be careful: We only
        recommend using this in combination with ``testmode=false``, since test mode orders can vanish at any time and
        you will not notice it using this method.
+   :query datetime created_since: Only return orders that have been created since the given date.
    :param organizer: The ``slug`` field of the organizer to fetch
    :param event: The ``slug`` field of the event to fetch
    :resheader X-Page-Generated: The server time at the beginning of the operation. If you're using this API to fetch

src/pretix/api/views/order.py

@@ -50,6 +50,7 @@ class OrderFilter(FilterSet):
     code = django_filters.CharFilter(field_name='code', lookup_expr='iexact')
     status = django_filters.CharFilter(field_name='status', lookup_expr='iexact')
     modified_since = django_filters.IsoDateTimeFilter(field_name='last_modified', lookup_expr='gte')
+    created_since = django_filters.IsoDateTimeFilter(field_name='datetime', lookup_expr='gte')
 
     class Meta:
         model = Order