data validation

[Donai]

Hi I have two cells which house dates, in cell a1 and b1, i want a data validation which will produce an error message if the user inputs the same date twice in any of those cells. So if a1 had 20/03/2011 and the user inputs the same date in b1 or vice versa then return a error message.

 

[Andrew Poulsom]

Try the custom formula:

=COUNTIF($A1:$B1,A1)=1

 

[Donai]

Try the custom formula:

=COUNTIF($A1:$B1,A1)=1

Thanks Andrew