If you know for sure that the date is always in YYYYMMDD such as 20160516 instead of 2106516 then it is very simple. Just put this in the cell that you want assuming A1 is where you have 20131218.
=DATE(LEFT(TEXT(A1,0),4), RIGHT(LEFT(TEXT(A1,0),6),2), RIGHT(TEXT(A1,0),2))
There could simpler methods but this will teach you the use of the Date, Left, Right and Text functions too.