With the value 20010129 in cell A1, enter this formula in B1:
=DATEVALUE(MID(A1,5,2)&"/"&RIGHT(A1,2)&"/"&LEFT(A1,4))

and the format the cell as a date. The various functions parse the
20010129 string into its components, and the DATEVALUE converts that
string to a datevalue that Excel can recognize.