Can't get modified_since param in /lists/listId/contacts to work
I'm trying to check if any contacts within a certain contact list were changed. I'm using /lists/listId/contacts v2 API method, providing listId and modified_since parameters. The modified_since parameter doesn't seem to be working, whether I provide it or not the call returns a full list of recipients, ignoring the modified_since option. For example I provide 2014-08-28T13:21:32.000Z as modified_since and the very first record returned has a "modified_date": "2014-08-27T13:21:32.000Z" property.
I thought I'm setting the date wrong, but regular /contacts call, with the same modified_since date set, returns a correct set of a few changed contacts.
It was returning an empty set, before I changed some of the contact list records just to be sure, after that it returns the whole list. Just a guess but maybe it takes list last update instead of contact?