I can help you with a formula, but the issue will be that Today keeps changing every day, so that date will change each day to the current date, which is not what you want.
If all of the data entry is done on a form, then I suggest that you make a Dynamic Form Rule
When multiple conditions are true
All of
when [Completed] is checked
when [date completed] is blank
Action
Change the value of date completed to the current date.
and uncheck he box at the bottom
However, if you are entering data in Grid Edit mode, then Dynamic Form Rules do not apply and you would need to set up an Automation or Pipeline to do this.
------------------------------
Mark Shnier (YQC)
mark.shnier@gmail.com
------------------------------