code: function lastNextDay(date, modifier, day, base_date) {
  
  		var current_iso_day,
  		    current_day,
  		    iso_day,
  		    diff,
  		    nr;
  
  		// Get the day count
  		nr = days[day];
  
  		// Get the current day
  		current_day = date.getDay();
  
  		// Calculate the difference
  		diff = nr - current_day;
  
  		if (diff == 0) {
  			diff = 7 * modifier;
  		} else if (diff > 0 && modifier < 0) {
  			diff -= 7;
  		} else if (diff < 0 && modifier > 0) {
  			diff += 7;
  		} else if (diff && modifier == 0) {
  
  			// Sunday is a special case
  			if (current_day == 0) {
  				diff -= 7;
  			}
  		}
  
  		diff = date.getDate() + diff;
  
  		date.setDate(diff);
  
  		return date;
  	}
docs: Go to a last/next day
  
  @author   Jelle De Loecker   <jelle@develry.be>
  @since    0.5.7
  @version  0.5.7
  
  @param    {Date}     date
  @param    {String}   modifier   -1 for last, +1 for next
  @param    {String}   day
  @param    {Date}     base_date
  
  @return   {Date}