three o'clock
`` In spherical trigonometry , '' he concluded , `` to solve the problem three elements must be known . I know four . Therefore , I can take each of the known , treat it as unknown , and have four ways to check my result . I find that the time might have been either three o'clock , twenty-one minutes and twelve seconds in the afternoon , or 3:21:31 or 3:21:29 , or 3:21:33 . The average is 3 : 21:26 and there can be no appreciable error except for a few seconds . I tell you that to show you how close I can come . The important thing , however , is that the date must have been one of two days , either May 22 or July 22 .