CONVERT takes a style parameter which can be used to shape the output of a date-datatype to string-datatype conversion - which is the usage that most folks are familiar with. It works for input too, describing the shape of a text string for a string-datatype to date-datatype conversion.
Your data corresponds to style 12: yymmdd. Hence, all you need to convert your data is this:
CONVERT(DATE,LEFT(MyField,6),12).
For fast, accurate and documented assistance in answering your questions, please read this article.
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