Using for-each-group with dates

I'm trying to use the for-each-group functionality, and group by a date, order by that date, and then display lines with the times.

Date Field:

<DATETIME>2024-11-17T09:30:00+00:00</DATETIME>

I've tried multiple incarnations of formatting a datetime for the grouping, and its either not grouping, or I'm getting errors.

<?for-each-group:current-group();xdoxslt:format_date:DATETIME;'mm-dd'?>

Does not group

<?for-each-group:current-group();format_date(DATETIME;'mm-dd')?>

Does not group

<?for-each-group:current-group();xdofx:format_date(DATETIME,'mm-dd')?>

Error:
<Line 54, Column 158>: XML-22008: (Error) Namespace prefix 'xdofx' used but not declared.
@Line 54 ==> <xsl:for-each-group select="current-group()" group-by="xdoxslt:grpby(xdofx:format_date((./DATETIME)[1], 'mm-dd'))">