This may not be a real XSLT question but more of an Umbraco question.
This function as written will output another format as expexted:
<xsl:value-of select="umbraco.library:FormatDateTime(data [@alias = 'kursusgangTilmeldingsfrist'], 'dd/MM/yyyy')"/>
When called with these three "second arguments" on the current date
'dd MM yyyy'
'dd-MM-yyyy'
'dd/MM/yyyy'
Then output will be:
10 02 2009
10-02-2009
10-02-2009 (and this should have been with "/" and not a "-")
Br. Anders