format

You can try

=Date('42736','MMM-YY')

Try this

=Date(Num#(date))

And attached also

your date field is a string rather a number.

could you provide the file "Additional report.xlsx" ?

For month name

=Date(Date(Num#(date)),'MMM-YY')

Or

=MonthName(Date(Num#(date)))

How do you want to it to be be?

Try:

MonthName(Date#(MONTH,'MMM-YYYY')) as monthName

or

MonthName(Date(Num#(MONTH))) as monthName

may be--

Date(Datefield,'MMM-YY')

ex:

Date('42736','MMM-YY')

It will give  Jan-17,Feb17...