Hi Trevor

Yes, I did it try with all kinds of periods and it doesn't work on
greater than 12. I'm not sure what could be the trouble as I tried both
cutting and pasting the formula as you wrote it here and typing it in
myself. I'm using Excel 2003 on XP if that makes a difference. But
the formula that daddylonglegs just gave works well for me.

Thanks for your help.