Accepted answer

this is caused by data-grouping feature. you can disable it or define a custom format for each time span.

  series: [{
    data: data,
    datagrouping: {
      enabled: false,

live demo:


api reference:


it seems there is a workaround using fooltip.formatter which allows to define a tooltip for both the series and the dates using a callback function which returns an array of the form [date, y1, y2, ...].

tooltip: {
    formatter: _ => ['test custom date', 'custom y']

in my case, i now reuse the default formatter for the y value but replace the logic to format the dates

tooltip: {
    formatter: function(tooltip) {
         let tt =, tooltip);
         tt[0] = 'test custom date';
         return tt;

live example:

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