Prof. Beckman has kindly pointed out to me that according to Are’s Islendingabók, ch. 7 (þá vas þat mælt et næsta sumar áþr i lǫgum, at menn scyllde svá coma til alþinges, es X vicor være af sumre, en þangat til quómo vico fyrr), the Althing in the year 999 A. D. was decreed for the time when ten (instead of nine) weeks of the summer had passed, i. e. it was postponed until a week later in the calendar. The reason for this is undoubtedly that the calendar (the week-year), and with it the Althing, had contrived to antedate itself a little more than a week in relation to the natural year, after Torsten Surt’s reform of the calendar had been introduced about the year 965. Here therefore we have an example of the empirical and occasional correction of the Icelandic calendar which was postulated above.