Right, reason for it being 1881 is due to the calendar on Jo's desk and DD being a wizard of the month!
I think there is a mistake somewhere because it says in wikia that DD fought Grindelwald in 1945, which would have been the year after Tom left school, but DD had waited 5 years before he confronted Grindelwald after meeting him in Godric's Hollow when DD was about 18 but that would make DD, 23 when he faced Grindelwald in 1945. DD would have been 22 when Riddle left hogwarts and still at hogwarts when he was supposed to recruit Tom to hogwarts.