How to convert dates (formatted as numbers) to real dates?

How can I convert many cells with “dates” that is writen and looks like this 20060323 (yyyymmdd) but have the format in calc as “standard numbers” to “real dates” format (like yyyy-mm-dd)? (So I later can make datedif on them)

If I try to change format for (the number) 20060308 to date with format yyyy-mm-dd it just gets transformed to -8713-04-03 instead of a real date format like 2006-03-08 which I want so I can do datedif.

(Have tryed to create a new date format as yyyyymmdd and format alot of empty cells with it and then paste all the numbers (like 20060323) in to them but that did not work either.)

Use Data-Text to Columns... feature, and assign “Date (YMD)” format to the column in question.