It sounds like you want to know when a record is marked as complete. Airtable formulas are constantly recalculated whenever any of their inputs change, so using NOW will not “lock in” the time of the change. NOW shows approximately the current time (but may be up to an hour late), depending on if the base is open or not.
Try using LAST_MODIFIED_TIME instead.
Status = "Completed",
You may also need to into the formatting options of the formula field to make sure that you are using the timezone and date format you want.