i don't think you can do it within the same column. there'd be no way
for an excel formula to compare & recognize a new date.
IMHO you'd need a helper column.......
dates are in colum a
formula in cell b1 would be:
=a1
formula in cell b2 would be:
=IF((A1=A2),A1,A2)
formula in cell b3 would be:
=IF((A2=A3),A2,A3)