UNIX_TIMESTAMP Error when using Tank Auth with MSSQL Server.

[eluser]afro[/eluser]
The UNIX_TIMESTAMP function is not recognized in msssql the server that am using,
can someone help me on how to go round this problem.

Code:

function purge_na($expire_period = 172800)
{
  $this->db->where('activated', 0);
  $this->db->where('UNIX_TIMESTAMP(created) <', time() - $expire_period);
  $this->db->delete($this->table_name);
}