static buildTimeString(displayTime, showHour) {
    const d = Math.floor(displayTime / 86400);
    const h = Math.floor((displayTime % 86400) / 3600);
    const m = Math.floor((displayTime / 60) % 60);
    let s = Math.floor(displayTime % 60);
    if (s < 10) {
      s = '0' + s;
    }
    let text = m + ':' + s;
    if (showHour) {
      if (m < 10) {
        text = '0' + text;
      }
      text = h + ':' + text;
      if (d > 0) {
        if (h < 10) {
          text = '0' + text;
        }
        text = d + ':' + text;
      }
    }
    return text;
  }
Builds a time string, e.g., 01:04:23, from |displayTime|.

@param {number} displayTime (in seconds)
@param {boolean} showHour
@return {string}