--- archive/archive.module 
+++ archive_new/archive.module 
@@ -356,6 +356,15 @@
   // the entire month into the block.
   $prev_month = $month==1?12:$month-1;
   $prev_year = $month==1?$year-1:$year;
+ //some old php versions don't have cal_days_in_month(), this code accounts for that
+ if(!function_exists('cal_days_in_month')){
+ 	function cal_days_in_month($calendar,$month, $year)
+		{
+		// $calendar just gets ignored, assume gregorian
+		// calculate number of days in a month
+		return $month == 2 ? ($year % 4 ? 28 : ($year % 100 ? 29 : ($year % 400 ? 28 : 29))) : (($month - 1) % 7 % 2 ? 30 : 31);
+		} 
+	  }
   $num_days_prev = cal_days_in_month(CAL_GREGORIAN, $month-1, $year);
   $next_month = $month==12?1:$month+1;
   $next_year = $month==12?$year+1:$year;