Modified implies that the item's record was created, updated, or marked Inactive. This parameter takes a UNIX-style timestamp, which we define as "Milliseconds since the Epoch." The datetime value is a UTC date and time. For example, if I have a UTC timestamp for Fri, 06 Nov 2015 18:17:56 GMT, the milliseconds value would be 1446833876000. Therefore, adding this parameter to the API search ("modifiedSince": "1446833876000") would return a store's items that were changed since November 6, 2015 at 18:17:56 UTC.