Not sure if this can be done without vba (preferred)

If a1,a21,a41 have a value greater than 0, I wish to stamp cell b1,b21,b41 and so on with todays date, but without the stamped value changing/advancing tomorrow. At the moment cells b1,b21,b41 are copied down as follows. =if(a1>0,TODAY(),"") This works fine, but the date stamp of course changes tomorrow. I can not apply code to the whole b column as cells b2:b20, b22:40 etc have other non-date format data to which the code does not need to apply - the todays date value cell occurs every 20 cells.

Thanks in anticipation.