Gets the difference between two dates and a timeline of a series of dates

diff-between-two-dates.js

export function diffBetweenTwoDates(start, end) {
  const startTime = new Date(start).valueOf();
  const endTime = new Date(end).valueOf();
  if (Number.isNaN(startTime)) throw new Error("Invalid start date");
  if (Number.isNaN(endTime)) throw new Error("Invalid end date");
  
  const fmt = (value, len = 2) => value.toString().padStart(len, "0");
  const diff = new Date(endTime - startTime);

  const diffNoTime = `${fmt(diff.getUTCFullYear(), 4)}-${fmt(diff.getUTCMonth() + 1)}-${fmt(diff.getUTCDate())}T00:00:00Z`;
  const days = new Date(diffNoTime).valueOf() / (24 * 3600 * 1000);

  const dayPart = days > 0 ? `${days}.` : "";  
  const timePart = `${fmt(diff.getUTCHours())}:${fmt(diff.getUTCMinutes())}:${fmt(diff.getUTCSeconds())}`;

  return dayPart + timePart;
}

get-timeline.js

import { diffBetweenTwoDates } from "./diff-between-two-dates";

export function getTimeline(...dates) {
  const timeline = [];
  if (dates.length < 2) throw new Error("At least two dates must be specified");

  const mDates = dates.map(x => new Date(x));
  timeline.push({
    date: mDates[0].toISOString(),
    cSpan: '00:00:00',
    tSpan: '00:00:00'
  });
  for (let i = 1; i < mDates.length; ++i) {
    timeline.push({
      date: mDates[i].toISOString(),
      cSpan: diffBetweenTwoDates(mDates[i - 1], mDates[i]),
      tSpan: diffBetweenTwoDates(mDates[0], mDates[i])
    });
  }

  return timeline;
}