code: function formatTimeCallback(seconds, pxPerSec) {
      seconds = Number(seconds);
      var minutes = Math.floor(seconds / 60);
      seconds = seconds % 60;
  
      // fill up seconds with zeroes
      var secondsStr = Math.round(seconds).toString();
      if (pxPerSec >= 25 * 10) {
          secondsStr = seconds.toFixed(2);
      } else if (pxPerSec >= 25 * 1) {
          secondsStr = seconds.toFixed(1);
      }
  
      if (minutes > 0) {
          if (seconds < 10) {
              secondsStr = '0' + secondsStr;
          }
          return `${minutes}:${secondsStr}`;
      }
      return secondsStr;
  }
docs: Use formatTimeCallback to style the notch labels as you wish, such
  as with more detail as the number of pixels per second increases.
  
  Here we format as M:SS.frac, with M suppressed for times < 1 minute,
  and frac having 0, 1, or 2 digits as the zoom increases.
  
  Note that if you override the default function, you'll almost
  certainly want to override timeInterval, primaryLabelInterval and/or
  secondaryLabelInterval so they all work together.
  
  @param: seconds
  @param: pxPerSec