In 2025 moment.js is still the best Date/Time Javascript package

I used moment.js frequently between 2017 and 2022, and it worked well for all my use cases. After 2022, I started using Day.js, a lightweight library designed to replace moment.js, with a size of just 2KB. In my experience, it has worked well in most cases and hasn’t caused any major issues—until this week.

This week, I encountered a serious issue with Day.js. The feature in question involves copying last week’s calendar events to the current week. However, March 9th marks the Daylight Saving Time (DST) transition. As a result, my newly copied events were shifted by one hour.

Here’s the problematic code:

const timeZone = "America/New_York";

const sourceDay = dayjs.tz(
  "2025-11-01 00:30:00",
  "YYYY-MM-DD HH:mm:ss",
  timeZone
);
console.log(sourceDay.toISOString());

console.log(sourceDay.add(7, "d").toISOString());

const targetDay = dayjs.tz(
  "2025-11-08 00:30:00",
  "YYYY-MM-DD HH:mm:ss",
  timeZone
);
console.log(targetDay.toISOString());

And here’s the output:

2025-11-01T04:30:00.000Z
2025-11-08T04:30:00.000Z
2025-11-08T05:30:00.000Z

After testing, I confirmed that this is a Day.js bug. Upon searching the issue tracker, I found that someone had already reported it back in 2021, yet it remains unfixed after four years.

Then, I became curious to see if moment.js or luxon had the same issue. After testing, I found that both handled the DST transition correctly without any problems.

This confirmed that the bug is specific to Day.js, and it’s not just an edge case—it directly affects time calculations when working with recurring events across DST changes.

At this point, I started wondering: should I continue using Day.js, or is it time to switch back to moment.js or luxon? While Day.js is lightweight and generally reliable, a critical bug like this—one that has remained unfixed for years—makes me question its long-term reliability.

For now, I’ve decided to temporarily work around the issue by manually adjusting for the DST offset, but this experience has made me rethink my choice of date-time libraries.

Have you encountered similar issues with Day.js or other libraries? Let me know your thoughts!