SQL QUERY

Hello,
How can i write a query to check if data is not entered in the table for past 15 days.
TableA
Field : start_time (datatype nvarchar)

The field start_time has data like which has the current date an time  :
2013-08-06T06:18:10Z
2013-11-18T10:22:12Z
2013-11-27T12:56:18Z
2013-11-28T07:33:41Z
2013-11-28T07:42:31Z
2013-11-28T07:47:41Z
2013-11-28T07:49:41Z
2013-11-28T19:01:52Z
2014-12-09T08:47:17Z
2014-12-09T08:49:07Z
2014-12-09T08:51:07Z
2014-12-10T07:48:38Z
2014-12-10T08:03:08Z
2014-12-11T13:03:01Z
2014-12-11T13:11:17Z
2014-12-17T15:24:41Z

Cheers