We had a date bug that happened two times a year, and we didn't know, you might have it too 😱
Tags: javascript, opensource, webdev, bug
Published: 2023-09-11 12:08:00

## TL;DR
Novu's team encountered a significant bug affecting date calculations in their CI/CD pipelines, hindering all deployments. 

The issue arose from the date-fns library's addMonths and subMonths functions.

We fixed this by using addDays and subDays functions instead.   

![Panic Gif](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/7qewo5qx57k5z4g466ot.gif)
  
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## Novu: Open-source notification infrastructure 🚀

Just a quick background about us. Novu is an open-source notification infrastructure. We basically help to manage all the product notifications. It can be In-App (the bell icon like you have in the Dev Community - Websockets), Emails, SMSs and so on.

[![Novu Request Stars On Github](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/134zbnlfi42ui6wn828k.jpg)](https://github.com/novuhq/novu)

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## The Mindset

When working in software development, we're always prepared for bugs to crop up. 

Sometimes they're small, easy to identify, and quick to fix. 

Other times, they're like this year's candidate for our 'Bug Of The Year'. 

This was a bug so elusive and mysterious that it had us rummaging through our pipelines, questioning our code-base, and coming face-to-face with the intricacies of date manipulation.

## Problems, Different Problems, and More Problems

Our CI/CD pipelines were failing. Specifically, two tests which were blocking ALL new deployments. It was time to put on our detective hats 🕵️.

We dove into our commit history using [`git bisect`](https://git-scm.com/docs/git-bisect) however it offered us no insight. Git bisect took us back to commits that where over 6 months in the past, long before any of our newest changes to the system that would have caused this. Was this bug created at the very beginning of Novu?

However, we did have a clue. Our failing unit tests showed us that we had incorrect date calculations. 

## Gathering the Clues 💡

Strangely, the difference was just one day. 

``` javascript
const startDate = new Date("2023-08-31");
const oneMonthAhead = addMonths(startDate, 1);
const result = subMonths(oneMonthAhead, 1);
console.log(result);  // Expected: 31st of August, Reality: 30th of August
```

We also found that this does not happen on 31st July.
``` javascript
const startDate = new Date("2023-07-31");
const oneMonthAhead = addMonths(startDate, 1);
const result = subMonths(oneMonthAhead, 1);
console.log(result);  // Expected: 31st of July, Reality: 31th of July
```

But the bug shows up again January 31st.
``` javascript
const startDate = new Date("2023-01-31");
const oneMonthAhead = addMonths(startDate, 1);
const result = subMonths(oneMonthAhead, 1);
console.log(result);  // Expected: 31st of January, Reality: 28th of January
```

So this bug only happens when we add 1 month to a month that has more days then the next month and then subtract 1 month to go back to the month before. 

## This is a sneaky one

So here is what we know so far:
- It would only show up on systems that does this specific sequence of logic.
- The code would have to be ran on one of the few dates that are effected. 
- This effect is not documented anywhere on any of the libraries we use.

The worst thing is that this bug is also shows up HR tools, finance tools, salary tools, public government tools all rely on this package but unfortunately it is still better then us making the functions our-self's.

It has been said many times that date-times are among the trickiest aspects of programming, and our current predicament served as a hash reminder.

![This is tough GIF](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/6xt36eh8300agqnklq7o.gif)

## Why a simple actions can lead to bad things

After finding this out, we had a 'Eureka!' moment. 
Our CTO, Dima Grossman, then had the idea to try it it on [raycast](raycast.com). Interestingly enough it was happening in their product too.

![Showing Raycast also uses date-fns](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/kw6sb2bfvl2nn6qwtfca.png)

![Mind Blown Gif](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/4aso2414apy6vz0svoar.gif)

We realized that the issue stemmed from being on the last day of the month, but what exactly was going awry?

## The Culprit: 

![date-fns icon](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/kc12kgdwa1q3hhw8n03l.png)

This popular utility library for date operations was at the heart of the problem.

Specifically, the `addMonths` and `subMonths` functions.

The `addMonths` function, when adding a month to the last day of any given month, would take you to the last day of the following month. Logical, right? 

``` javascript
// source: https://github.com/date-fns/date-fns/blob/main/src/addMonths/index.ts
const daysInMonth = endOfDesiredMonth.getDate()
  if (dayOfMonth >= daysInMonth) {
    // If we're already at the end of the month, then this is the correct date
    // and we're done.
    return endOfDesiredMonth
  } else {
    // Otherwise, we now know that setting the original day-of-month value won't
    // cause an overflow, so set the desired day-of-month. Note that we can't
    // just set the date of `endOfDesiredMonth` because that object may have had
    // its time changed in the unusual case where where a DST transition was on
    // the last day of the month and its local time was in the hour skipped or
    // repeated next to a DST transition.  So we use `date` instead which is
    // guaranteed to still have the original time.
    _date.setFullYear(
      endOfDesiredMonth.getFullYear(),
      endOfDesiredMonth.getMonth(),
      dayOfMonth
    )
    return _date
  }
```

But the `subMonths` function, rather than having its own dedicated logic, simply reused `addMonths` with a negative number. D.R.Y principles in action, but with an unintended consequence.

```javascript
// source: https://github.com/date-fns/date-fns/blob/main/src/subMonths/index.ts
export default function subMonths<DateType extends Date>(
  date: DateType | number,
  amount: number
): DateType {
  return addMonths(date, -amount)
}
```
## Here is what exactly caused our issue  

Let's put it this way:
- For 28th February, add one month and then subtract one month, and you get 28th February. No problems there.
- But, for 31st August, add one month and then subtract one month, and you land on... 30th August. That's one day lost in date limbo!

The core of the issue was the way `addMonths` determined the end of the desired month.

For days that were not at the end of the month, the logic was sound. 

However, for the last day of a month, the function defaulted to the end of the next month instead of adding the correct amount of days.

## The Simple Fix

To ensure a consistent approach to date manipulation, we shifted from using `addMonths` and `subMonths` to `addDays` and `subDays`. 

![Quicly Coding Cat Gif](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/2aoc0bfp131dc79q5dor.gif)

This provided a more granular and precise way to handle date calculations, and importantly, allowed us to sidestep the `addMonths` pitfall.

## Lessons Learnt

This bug served as a strong lesson in a few key areas:

1. **Assumptions are Risky:** Never assume that widely-used libraries are infallible. Even the most popular ones have their quirks.
2. **Tests are Gold:** If not for our rigorous testing suite, this bug might have remained hidden, only to wreak havoc at the most inopportune moment.
3. **Dates are Tricky:** They've always been, and will continue to be, a challenging aspect of software development. Always handle with care.

While this bug threw a wrench in our pipes, it also reinforced the importance of comprehensive tests and the need to continually question and challenge our assumptions. 

## Death of this Bug

In a world of code where dates and times form such a crucial part of our applications, bugs like these provide not just a hiccup, but a learning opportunity. The next time you find a weird issue in your application, dig deep. Who knows, you might just uncover the next 'Bug Of The Year'.

![Bug Goodbye Gif](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/0ps74vqneyl5oxnqmhxj.gif)

You can find the PRs and Issues here:
- [Novu PR](https://github.com/novuhq/novu/pull/4071)
- [Issue on date-fns](https://github.com/date-fns/date-fns/issues/3506)