Checkmark in one column to show a completed date on a second column and it stays that date

edited 12/09/19 in Smartsheet Basics

I am looking for a formula that when I checkmark a box (to show a task completed) the column next to the checkmark will auto populate the date the box was checkmarked. I also want it to stay that date unless that row or checkbox is modified/unchecked.  

I got as far as the date populating but the next day it is still the current days date rather than the previous day on which the box was checked. 

any help and advice would be appreciated.