Not sure this is a bug - and not necessarily limited to Swedish locale.
DayName ( 2014-12-12 ) = Tuesday but DayName ( "2014-12-12" ) = Friday
GetAsDate ( 2014-12-12 ) = GetAsDate ( 1990 ) = June 13 of Year 6. Which I guess may very well have been a Tuesday.
You are correct, David.
Not obvious at first, but 2014-12-12 is not a Swedish date in a variable, and GetAsDate(2014-12-12) is date 0006-06-13, but Getasdate("2014-12-12") is date 2014-12-12. Nevertheless risky business, unless it's a pure date field.