You need at least yyyy mm dd to make sorting work. The column is sorted by the string you are seeing, not the real date underneath it, so the order of the date components is important.
Try
Code:
{:'re(list_item(formats_modtimes('yyyy-MM-dd'), 0, ','), '^.*?:', '')'}
Take out or replace the - characters if you don't like them.
And you are welcome.