Extract characters from a cell

Dear all, kind regards

I am working on some data and I need to extract from column A:A exactly only the dates that start in 2017 and end in number, that is to say with a length of 8 characters [20171015], the problem is that I am performing this operation in column C:C but due to the extension of some characters in some rows it does not bring me the totality of numbers that I am requesting. Is there an additional method to make this possible or can I fix the formula that I am using in a wrong way?

I appreciate your collaboration.

PS: the formula I am using is
=MID(A9,FIND(" ",A9,B9+1),9)

You may do not need column B
If year is exact 2017:
=MID(A2,SEARCH(" 2017",A2)+1,8)
Or year that >2000
=MID(A2,SEARCH(" 20",A2)+1,8)
Or, more generic, extract first 8-digit number:
=AGGREGATE(14,6,MID(A2,ROW(INDIRECT("1:"&LEN(A2))),8)+0,1)