code: public static function unixToJd($tstamp)
  	{
  		list($year, $month, $day) = explode(',', date('Y,m,d', $tstamp));
  
  		// Make year a positive number
  		$year += ($year < 0 ? 4801 : 4800);
  
  		// Adjust the start of the year
  		if ($month > 2)
  		{
  			$month -= 3;
  		}
  		else
  		{
  			$month += 9;
  			--$year;
  		}
  
  		$sdn  = floor((floor($year / 100) * 146097) / 4);
  		$sdn += floor((($year % 100) * 1461) / 4);
  		$sdn += floor(($month * 153 + 2) / 5);
  		$sdn += $day - 32045;
  
  		return $sdn;
  	}
docs: Convert a UNIX timestamp to a Julian day
  
  @param integer $tstamp
  
  @return integer