The on-sale date is the publication date reported in the U. S. Copyright Office filing.
Cover colors credit from Greg Theakston, via the GCD Error List.
Inside front cover.
First appearance of TNT & Dan (appears next in World's Finest Comics #5). Order of TNT appearances determined by copyright dates (Star Spangled #7 on Feb 1st 1942, World's Finest #5 on Feb 3rd 1942). Note that the two books essentially debuted at the same time.