code: function (point) {
  		var series = this,
  			tooltipOptions = series.tooltipOptions,
  			xDateFormat = tooltipOptions.xDateFormat,
  			dateTimeLabelFormats = tooltipOptions.dateTimeLabelFormats,
  			xAxis = series.xAxis,
  			isDateTime = xAxis && xAxis.options.type === 'datetime',
  			headerFormat = tooltipOptions.headerFormat,
  			closestPointRange = xAxis && xAxis.closestPointRange,
  			n;
  			
  		// Guess the best date format based on the closest point distance (#568)
  		if (isDateTime && !xDateFormat) {
  			if (closestPointRange) {
  				for (n in timeUnits) {
  					if (timeUnits[n] >= closestPointRange) {
  						xDateFormat = dateTimeLabelFormats[n];
  						break;
  					}
  				}
  			} else {
  				xDateFormat = dateTimeLabelFormats.day;
  			}
  		}
  		
  		// Insert the header date format if any
  		if (isDateTime && xDateFormat && isNumber(point.key)) {
  			headerFormat = headerFormat.replace('{point.key}', '{point.key:' + xDateFormat + '}');
  		}
  		
  		return format(headerFormat, {
  			point: point,
  			series: series
  		});
  	}
docs: Format the header of the tooltip