GtkCalendar::get_date
get_date() returns the currently selected calendar date as an array. The contents are the year (e.g. 2002), the month (1 - 12), and the day (1 - 31), in that order.
The range for the month is 1 - 12 here, but 0 - 11 in the select_month() method. Marked dates also run from 0. Beware!
忘却曲線を使ってこの知識を確実に記憶に残す
