Then it sounds like you will need to write a plugin to define the variables you want to use. You'll probably want to use one of the postbit_* hook locations. You should look in the code to see what variables are available at each location. If you have a date available, then you should be able to format it using something like (this is taken from a mod I wrote and you'll have to change it a little bit, I'm sure):
PHP Code:
$user['date'] = vbdate($this->registry->options['dateformat'],$user['dateline']);
$user['time'] = vbdate($this->registry->options['timeformat'], $user['dateline']);
$userdateline = $user['date'] .' '. $user['time'];
And then you would use $userdateline in the template. As I said though, take a look in the code around the hook location to see what the names of the variables are that you would use in your plugin.