Hi ,
I doubt that what you want can be done through a cell format , since cell formats do not change the contents of a cell , but only change the way it is displayed to the user.
If you want a date such as Jan 17 , the cell needs to contain the numeric value between 42736 and 42766.
If your cell contains a value such as 201701 , this needs to be converted to a value between the above two values , only then can a cell format be used to display Jan 17.
Assuming your value 201701 is in cell C1 , the following formula will do this :
=DATE(LEFT(C1,4), RIGHT(C1,2), 1)
Format the cell as mmm yy.
Narayan