What is the data type of the column dt? You can find out using:
SELECT data_type FROM INFORMATION_SCHEMA.[COLUMNS]
WHERE tablename = 'tb' AND column_name = 'dt'If the data type is a datetime type (datetime, datetime2, smalldatetime etc.), it stores the date and time part - the fact that you are not seeing time in the first case is just an artifact of the display. If you do want to display it in a specific format, you can convert it the desired string format - see here: http://msdn.microsoft.com/en-us/library/ms187928.aspx
If it is not a datetime type, can you post what the data type is?