You are trying to store the value 18 in a month, which is not possible of course.

You can either use CONVERT to convert the string to a datetime. You only need to specify the correct format.

DECLARE @date DATETIME= CONVERT(DATETIME,'18-06-2014 11:38:46.050',105); -- apparently the Italian format

SELECT CAST(@date AS VARCHAR);

Or you can use SET DATEFORMAT to override the implicit date formatting.

SET DATEFORMAT dmy;

DECLARE @date DATETIME= '18-06-2014 11:38:46.050';

SELECT CAST(@date AS VARCHAR);