[3.1.4 modification] modify calendar library, add extra column before calendar

if you want a quick and dirty method like me, you can do the following:
the mentiond line numbers represent the original line numbers from the original calendar.php file. that is why the changes are in reverse order, from the bottom up:
Copy the system/libraries/calendar.php file to Application/libraries/MY_calendar.php
Go to line 526 and replace it with the following text:

PHP Code:

foreach (array('table_open', 'table_close', 'heading_row_start', 'heading_previous_cell', 'heading_title_cell', 'heading_next_cell', 'heading_row_end', 'week_row_start', 'week_row_before_cell', 'week_day_cell', 'week_row_end', 'cal_row_start', 'cal_row_before', 'cal_cell_start', 'cal_cell_content', 'cal_cell_no_content', 'cal_cell_blank', 'cal_cell_end', 'cal_row_end', 'cal_cell_start_today', 'cal_cell_content_today', 'cal_cell_no_content_today', 'cal_cell_end_today', 'cal_cell_start_other', 'cal_cell_other', 'cal_cell_end_other') as $val) 


Go to line 336 and paste the following before the line already there

PHP Code:

           if (array_key_exists('cal_row_before', $this->replacements))
            {
                $maxday = $day-1;
                if ($maxday >= 32)
                {
                    $maxday = max(array_keys($data));
                }
                $out = str_replace('{to_day}', $maxday, $out);
            } 


Go to line 297 and paste the following before the line already there

PHP Code:

                       if (array_key_exists('cal_row_before', $this->replacements))
                        {
                            $out = str_replace('{from_day}', $day, $out);
                        } 


Go to line 284 and paste the following before the line already there

PHP Code:

           if (array_key_exists('cal_row_before', $this->replacements))
            {
                $out .= $this->replacements['cal_row_before'];
            } 


Go to line 271 and paste the following before the line already there

PHP Code:

        if (array_key_exists('week_row_before_cell', $this->replacements))
        {
            $out .= $this->replacements['week_row_before_cell'];
        } 


Replace line 51 with the line below

PHP Code:

class MY_Calendar extends CI_Calendar { 


Now just call your extra calendar like below

PHP Code:

       $this->load->library('calendar', $prefs);
        $this->load->library('MY_calendar', $prefs); 


and just use the normal functions like you are used to.
Now you can use the following fields in your template (with examples included)

PHP Code:

           {week_row_before_cell}<td>&nbsp;</td>{/week_row_before_cell}
            {cal_row_before}<td>{from_day}-{to_day}</td>{/cal_row_before} 


The field week_row_before is to accommodate the extra column in the week name row
The field cal_row_before is the extra cell before each week row. In this field, you can use the fields from_day and to_day to use the first and last day of that week in your extra cell.
View a demo (partial screenshot) below: