I cannot think of a way to do it that does not involve either VBA or
helper columns. The technique I am suggesting here will involve
creating a column with the individual dates, so that you can then get
the maximum. Assuming the dates are in A1.

In B1:
=FIND(CHAR(10),A1)
In B2:
=FIND(CHAR(10), $A$1, B1+1)
In C1:
=DATE(YEAR(VALUE(LEFT(A1,B1-1))), MONTH(VALUE(LEFT(A1,B1-1))),
DAY(VALUE(LEFT(A1,B1-1))))
In C2:
=DATE(YEAR(VALUE(MID($A$1,B1+1,B2-B1-1))),
MONTH(VALUE(MID($A$1,B1+1,B2-B1-1))),
DAY(VALUE(MID($A$1,B1+1,B2-B1-1))))

Copy down the formulas in B2 and C2 until you get #VALUE!. You can then
get the max of column C:C.

HTH
Kostis Vezerides