If SQL Server is conveting a string to DATETIME, there are only 2 unambiguous formats that SQL Server will always parse correctly:

'YYYYMMDD HH:MM:SS:mmm', example: '20100617 15:46:59.997'

'YYYY-MM-DDTHH:MM:SS:mmm', example: '2010-06-17T15:46:59.997'

Any other format can have problems, depending on the settings of DATEFORMAT and LANGUAGE.