I did the peculiar experiment of picking up the calendar available in the
panel bottom-right and clicked myself backwards to the year 1753. I found
that the calendar does not properly reflect the switch from julian to
gregorian calendar in Sweden the year 1753, february, which had only 17 days.

In fact, it appears Sweden always had a gregorian calendar. The structure
of the actual Swedish calendar is outlined at this web page:

http://susning.nu/Gregorianska_kalendern

If you can help me in pointing out in which part of GNOME this calendar
thing sits, and where it may be modified, I can help out in fixing it for
Swedish locale. I think the GNU version of the unic "cal" tool (most
distributions use the BSD "cal" however) recognize this pecularity.