Highcharts will automatically try to find the best format for the current zoom-range. This is done if the xAxis has the type 'datetime'. Next the unit of the current zoom is calculated, it could be one of: second minute hour day week month year This unit is then used find a format for the axis labels. The default patterns are: second: '%H:%M:%S', minute: '%H:%M', hour: '%H:%M', day: '%e. %b', week