Which formula did you try and where have you implemented that formula?
I used this:
=IF(ISERROR(DATEDIF(INDIRECT("A"&MATCH(REPT("z",255),B:B)),TODAY(),"D")),"",DATEDIF(INDIRECT("A"&MATCH(REPT("z",255),B:B)),TODAY(),"D"))
In cell "B43". Column "A" has the dates the tasks in columns "B" thru "H" have the check marks in. And I might mention it works fine if I put it in any other column.
[TABLE="width: 599"]
<tbody>[TR]
[TD="class: xl63, width: 65, bgcolor: green, align: center"]Date[/TD]
[TD="class: xl63, width: 65, bgcolor: green, align: center"]Mow[/TD]
[TD="class: xl63, width: 65, bgcolor: green, align: center"]Edge[/TD]
[TD="class: xl63, width: 65, bgcolor: green, align: center"]Weeds[/TD]
[TD="class: xl63, width: 65, bgcolor: green, align: center"]Leaves[/TD]
[TD="class: xl63, width: 65, bgcolor: green, align: center"]2, 4D[/TD]
[TD="class: xl63, width: 65, bgcolor: green, align: center"]Roundup[/TD]
[TD="class: xl63, width: 72, bgcolor: green, align: center"]Weed & Feed[/TD]
[TD="class: xl63, width: 72, bgcolor: green, align: center"]Seeded[/TD]
[/TR]
</tbody>[/TABLE]
Row 43 Has the days from last checked.
This formula is now in Cell "B43" and seems to be working fine:
=IF(SUMPRODUCT(MAX((B6:B41<>"")*ROW(B6:B41)))>0,DATEDIF(INDIRECT("A"&SUMPRODUCT(MAX((B6:B41<>"")*ROW(B6:B41)))),TODAY(),"D"),"")
Hope this explains it better.