I am assuming it was because your members registration date was in this format: Year - Month - Day? If so, you will need to play a small trick in it before sending to the mktime() function:
PHP Code:
//Lets assume your date is in this format: 2011-09-11
$date = "2011-09-11";
$datearray = explode("-",$date);
$timestamp = mktime(0,0,0,$datearray[1],$datearray[2],$datearray[0]);
This way you will be able to get timestamp to work, hopefully you can see what I am doing here.