I was given a formula (I don't understand it at all but it did what i needed at the time)
=IF(ISBLANK(F7)," ",INDEX($R$6:$R$59, MATCH(MIN(ABS($R$6:$R$59-$B$10)), ABS($R$6:$R$59-$B$10), 0)))
What it does ( I think) is look through a list of dates (R6:R59) for the nearest date to that in B10...