The bug exists every time today's date is greater than the lowest total number of days in a month (28). So it's a bug for 3 days in January, (sometimes 1 day in February,) 3 days in March, 2 days in April, etc.
in a SPECIFIC month. it's only 28 for february, and then only sometimes. it's only going to affect months where the month contains more days than the month after it. january, march, may, august, and october.
of those, however, there's more than one date in january which would error, so the total is more than 5. 6 or 7 depending on if it's a leap year
I think you did too, I don't have the slightest clue where your math came from. This bug happens when the month you're setting it to has fewer days than the month in the date object, so it rolls the day forward into the next month. It's the same functionality that lets you just add days and iterate over the whole year.
In non-leap years, there are potential invalid target months when the current date is January 29th, January 30th, January 31st, March 29th, March 30th, March 31st, April 29th, April 30th, May 29th, May 30th, May 31st, June 29th, June 30th, July 29th, July 30th, July 31st, August 29th, August 30th, August 31st, September 29th, September 30th, October 29th, October 30th, October 31st, November 29th, November 30th, December 29th, December 30th, or December 31st.
That is, the number of days of a given year where this bug applies are: 3 in January, 3 in March, 2 in April, 3 in May, 2 in June, 3 in July, 3 in August, 2 in September, 3 in October, 2 in November, and 3 in December.
Very relatedly, there's a potential similar footgun related to hours, due to the fact that dates initialized without hour default to 00:00, and that JS dates are in user's current local timezone.
It's possible then that after changing the month programmatically, the time component will jump to 23:00 due to timezone DST changes, and the date will jump back one day as well due to this (and in an unlucky case, the month will jump back as well!)
Example bug I had in 2014 when Russian timezone definitions have changed:
targetEndDate = new Date();
targetEndDate.setFullYear(
endDate.getFullYear(),
endDate.getMonth(),
endDate.getDate());
That works because setFullYear has a three argument form that takes the year, month, and date avoiding the inconsistency that can arise by setting those one at a time.
But you know what else has a three argument for that take the year, month, and date? The Date constructor.
So why not fix it like this?
targetEndDate = new Date(
endDate.getFullYear(),
endDate.getMonth(),
endDate.getDate());
Using the empty constructor would initial the object with the current date and time, but they promptly overwrite those.
The Date construction also has a form that takes another Date object, so I wonder if they could have simple used:
You should never, ever do date math naively like this. There are too many unexpected edge cases, especially between time zones or daylight savings time or leap years, but even without them: https://moment.github.io/luxon/#/math
In fact I would strongly argue you should never use the JS Date built-ins at all because they are terrible. Use a library like Luxon or date-fns. As a frontend dev, this is the most common category of bugs I've dealt with in my career, and I've spent dozens of hours fixing other people's datetime handling because of how poorly implemented the JS Date API is. It's full of minefields and gotchas.
The Temporal API is supposed to fix some issues, but that's been in development for like a decade now.
Reminder to use something like the date-fns library for immutable operations on dates. The native JS date library can be painful to work on when doing operations like adding or subtracting time.
A function that takes all the components at once can resolve this conundrum internally.
A function that takes a simple struct (containing each component) can also handle the conundrum. The function could of course even take a string representation.
To have a better understanding of what operations you are trying to do. I can't answer your question specifically because it depends a lot on use case and what date/time standards you want to use.
To expand on this a little, there are operations that have slightly ambiguous meanings/can end up with different answers depending on your assumptions.
e.g. If today is the 30th of January 2024. What is the date in 1 months time?
Should it be the 29th of February? (as the 30th doesn't exist). The 28th? (the penultimate day of the month, like the 30th of Jan). The 1st of March (a 'standard' month should be considered 30 days, so 30 days from now).
To be honest, use a library where someone else figured out the ambiguities and accounted for the edge cases. Good starting point: https://moment.github.io/luxon/#/math
Date-fns is fine for simpler use cases but Luxon is a lot more complete, especially where it comes to time zones.
This is not the kind of thing you just want to blindly hack your way through without a really thorough understanding of how different cultures/locales/governments handle dates and times.
If you have to do math across daylight savings time boundaries that change over time (because governments and regulations change), converted to two or more time zones (which again have their own separate DST rules), and possibly span some 30 or 45 minute time zones (they exist!) this will quickly get out of hand and unreadable. Not just unreadable, but incredibly difficult to reason about.
It gets even worse when the backend stores only the offset (as in the ISO time, like T23:00:00+08:00) because then you lose the original time zone and can't be sure whether it came from a DST zone or another country in the same zone (but only during part of the year).
When you cross DST switchovers, for example, you may magically gain or lose an hour. A naive milliseconds calculation will miss that. It gets worse because DST in a country isn't a static thing either, but will change with the laws over time. So over decades, you need to have several lookup tables.
And people are ambiguous. Is January 31 plus 1 month the end of February or the beginning of March? What about during a leap year?
And a "day" isn't necessarily 24 hours (because of DST, again). Humans doing day math across those boundaries will just ignore (or really, never think about) the missing or gained hours, but computers have to explicitly account for it.
Then once you throw in different time zones it gets even crazier, especially for the half hour and 45 minute zones.
It's okay to store datetimes in epoch milliseconds (ideally with a separate field for the time zone, not just offset, which holds less information). But you can't easily/correctly do math on that without more specific instructions and cultural/locale adjustments.
targetEndDate = new Date();
targetEndDate.setFullYear(endDate.getFullYear());
targetEndDate.setMonth(endDate.getMonth());
targetEndDate.setDate(endDate.getDate());
Even if the endDate is valid, this can fail, because today is the 31st, which doesn't exist in endDate's month.
If you just did:
targetEndDate = createDate(endDate.getFullYear(), endDate.getMonth(), endDate.getDate());
// createDate is an intentional placeholder, I didn't want to double-check the actual syntax
you wouldn't pass through the transitional stage where you have the new month but today's day of the month, which is what causes the bug.
In this case, using mutation on the individual fields makes you have to transition through an invalid state to get back to a valid one, and the JS date object does something unexpected (though I think there's nothing good to do here, an exception is probably the best you could do). So mutation really is at issue here. In general, mutation creates room for these kind of counter-intuitive state transitions to arise.
The direct "immutable" equivalent of the bug code (purely based on my experience with .NET Core immutable collections) would have the setX methods return a new immutable datetime with the field set as requested. So just "use an immutable type" wouldn't fix this bug.
"Change the code completely so you supply all 3 components at the same time" obviously fixes the bug, but that doesn't require an immutable type.
The .NET immutable collections seem to mostly avert problems like this. For example, ImmutableDictionary.Add<TKey, TValue>(key, value) throws an exception if key already exists with a different value (as determined by an explicit or implicit IEqualityComparer<TValue>).
.NET's (immutable) DateTime, on the other hand, has AddMonths(Int32), which sets the day to Min(original day number, last day of result month), which, while arguably reasonable, isn't obvious without reading the documentation.
My favorite counterintutive DateTime "mutator," however is AddMilliseconds(Double): prior to .NET 7, its floating-point argument is rounded to the nearest integer (!?!).
> mutation on the individual fields makes you have to transition through an invalid state
If you want to do this mutation-style for some reason, you should use the builder pattern (or whatever it's called). So in this case you'd instantiate a DateBuilder instance instead of a Date instance, and finally call .date() to get the actual Date instance from the builder instance.
Better yet, in this particular case, don't use the builder pattern, and instead just provide a sensible set of constructors with appropriate defaults for unspecified arguments (e.g., 00:00 for unspecified time).
For a simple date/time class, the builder pattern reeks of pointless overengineering.
I make this bug in my taxes every now and then. Most invoices are from EU, but the few US ones have the months in wrong order, I only notice if the days exceed 12
I've seen people do YYYY/DD/MM, so your friend's format isn't foolproof. OTOH although I'm sure some insane person somewhere has done YYYY-DD-MM I've never seen it so YYYY-MM-DD should be safe.
Some languages pronounce the date as year, day, month so some people write YYYY/DD/MM. Since both YYYY/DD/MM and YYYY/MM/DD make sense to different people, neither should be used.
I also really like this form for regular communications, though I use spaces. It feels more natural than ISO format while being unambiguous. You can also add the day of the month and it's nicely balanced between letters and numbers, and you can also drop the year when it's clear from context:
"Hey, are you free for dinner on Wed 23 Aug?"
ISO is great for records and filename, but it's clumsy in conversation:
Unlike many issues with dates, this one is purely a result of a wrong API design. It does not make sense to "set a component" of a date all by itself (unlike hours/minutes/seconds in a day), so an API that allows you to so is plain broken.
Correct API would be to set the components D,M,Y all at once and have them validated on the spot before returning a valid Date value.
I had something like this for a while. It wasn't a bug per-say, it was a test flake due to dates and follow on calculations based on them. On the plus side, it prevented you from working on the weekend as the tests/PRs would always fail during that time.
Yeah, there's multiple variations of that bug possible depending on what you set first, so setting these in that specific order likely avoided one bug they noticed.
They just didn't realize they have a different bug now.
