[kh99]

I think $thread[dateline] is the date of the first post, but it's not formatted as a string. You'd need to format it using something like

$thread['firstpostdate'] = vbdate($vbulletin->options['dateformat'], $thread['dateline']);

in a plugin (maybe hook location threadbit_display). Then you should be able to use $thread[firstpostdate] in your template.

(BTW, I haven't tried this at all).