LASTED UPDATE FIELD SETS EVERY ENTIRE COLUMN TO DAY THE SHEET WAS OPENED

I have a column on a sheet set to modified date but the date sets to the date/time the sheet was last opened rather than the date the row was updated in every cell in the column. Can anyone tell me what i am doing wrong please?

I want the date the actual row (any cell) was updated so i can set a reminder if it's not been updated in a while

Thanks