The questions do not make sense so stop trying to ask them. It’s like asking if 2022-01-01 is an orange.

Generally speaking you want context of what the date or datetime is being used for. If you want to know when Christmas takes place you will use a date: it does not take place on 2022-12-25 00:00:00 through 2022-12-25 11:59:59 because this would require a timezone and Christmas takes place at different UTC times around the globe. But you can reasonably say that Christmas takes place on 2022-12-25 and leave it at that to let the implementation of whatever program figure out if it is or is not currently Christmas based on the information it has about time and timezone.

The answer is no/no/no because you cannot define a general comparison operator between dates and times without additional context. It's possible that you can define a contextual comparison operator that works for your domain and the question you want to ask, but without knowing the application for this comparison it's pointless to try to make a general statement.

My mental model for these kinds of things is that Times are instants and Dates are either:

1. Ranges (with the start and end Time depending on TZ and possibly other context)

2. Discrete cells of a calendar (which are mostly TZ independent - July 5th doesn't happen at the same time everywhere, but it is well-defined everywhere)

Also, I've been writing code for 25 years and I still have no idea what a DateTime is.

I am a little shocked that neither the blog post nor the twitter discussion nor any of the discussion here clearly identifies the missteps of this approach beyond alluding to a "category" or "type" error (which is correct, but not particularly informative.)

The issue here is around the semantics of the mathematical operators. It isn't even really about the types to which they apply; there are systems where `=` is well defined on heterogenous types.

The reason the answer to all these questions is a clear "no" is that they do not satisfy the core properties of the operators. For example, take equality. Equality in almost all mathematical constructs means that it supports substitution, is symmetric, transitive and reflexive. There are also well defined properties for the concepts of `greater than` and `less than`.

So, no, the OP's conclusion "Literally we’re all just making it up" is incorrect. You cannot use the operators `=`, `<`, and `>` between dates and times because they do not satisfy the core properties that define those operators. (I guess you could try to document an alternate definition of equality without the symmetric property in your documentation but... good luck with that not leading to massive confusion.)

Where you can just make it up is to define new operators as you actually want them to be. It's not `=`, it's `myDateTime=()` and then you're free to write the definition of that yourself. As long as you're consistent in the UI of how you present it (don't pretend it's vanilla `=` to the user!) you will at least be telling the truth. It may not solve all your problems but at least you won't be feeding any more to the fires of confusion, which you will as long as you keep pretending it's possible to make `=` mean something that's not reflexive.

I deal with this on a regular basis, and my approach is straightforward: I don't compare them. If a have a date on one side and date-time on the other, then I am missing the data necessary to make a comparison. The question seems like a non-sequitur because the implied question is "how do you compare two variable when one of them does not have the data needed for comparison"

The only question in circumstance like this is whether or not my requirements allow for-- or can reasonably be modified-- to strip off time and only compare dates.

If time is an inherent requirement to the project then the response I give is simple: "Then begin collecting time data."

I hate to be "that guy" but I think everyone is looking at this wrong.

It's not a programming issue, a type casting issue or a CS issue.

It's a UI issue. That the point you got the date and the datetime values, there was a human being (either a user, an admin or a programmer) sitting in front of the screen and they were asked a question in some way.

The precise way that question was presented affected their intent. It's their intent you are asking about here. So you need to think about what they were asked and how they were asked it. If that was consistent then you can make a reasonable call here. If it's inconsistent at different times and places then the meaning of that date is different and you can't solve this in any reasonable way.

The CS answer: bottom, bottom and bottom.

The static typing answer: type error, type error and type error.

The dynamic typing answer: exception, exception and exception.

The philosophy answer: category error, category error and category error.

My favourite is battling with people who use 23:59:59.

Where I work (banks and other financial institutions) there is frequently a need to check whether something happened within a particular day. Or maybe select records from the database for a day, or do some other kind of logic or filtering.

For some unknown reason, most people decide that best way to do this is to take start date as the beginning of the period and then add to it 23 hours, 59 minutes and 59 seconds and use that as the end of the period.

Explanations that they are missing a whole second do not seem to be working. People are absolutely convinced they are doing this correctly.

I thought about this for a long time and I arrived at an explanation.

It seems that some people use the time as a label for a span of time of unit length. March 21st is a label for an entire day. 12 pm is a label for an entire hour that starts at 12pm and lasts an hour, etc. 2021 is a whole year.

And in some contexts it makes sense. Way say something happened on 12th of January -- we use Jan, 12th as a label for an entire day that we would otherwise have to denote with two timestamps. But in some contexts what we need is an exact point in time, a timestamp. And here is where a lot of people just don't think / can't recognise a difference between timestamps and labels for a span of time.

If you use a wrong model then yes, the day starts with a second labeled 00:00:00 and ends with a second labeled 23:59:59.

Except that's not how most of the underlying software works. Most software in this case expects two exact timestamps to denote the end of the span of time. And 23:59:59 is just 1 second shy of the actual end of day which means that, even if we are missing an entire second of the day, most of the time everything seems to work fine.

Unless you are large bank and you have millions of transactions all over the clock that have to accumulated exactly. Then yes, it makes a lot of difference.

Another explanation is that people don't seem to be comfortable with the concept of selecting items from between 00:00:00 of one day and 00:00:00 of the next because they are seeing another date.

No/No/No definitively. Dates without times are ranges. If one person is born on 2022-06-01 12:00:00 and another person was born on 2022-06-01 13:00:00 then they have the same birthday 2022-06-01. It follows, that if you know that 2 people are born on the same day such as 2022-06-01, then it is unknown if they were born on the same time. So adding a default time to a day (such as 00:00:00) is nonsensical.

Ultimately, this is the same problem as “is ‘04’ > 3”? That is to say, you can only properly compare two things of the same types, and you can either implicitly or explicitly cast.

The easiest option is that comparators should only work on the same data type, to avoid any ambiguity, leaving it up to the user to do explicit casting, and throwing errors if they don’t.

Of course, like integers and decimals, maybe it makes sense to have implicit casting, but it’s unintuitive to me if “‘2020-01-01’ > ‘2020-01-01 12:000’” should be cast to two dates, two timestamps, or two timestamptzs. Even if a language allows implicit casting, it’s probably an area where not doing it as a author is a smell.

I only read the question and am not aware what domain/language this is coming from.

It seems they’re talking about Instants with different resolutions.

My answers are no, no, and yes.

If we see what they’re calling date as integers and times as floats, the questions become

Is 4 equal to 4.5? No

Is 4 between 4.5 and 200.5? No

Is 4.5 after 4? Yes

How about this: a date, or indeed even a time, is either of the shortest unit used in calculation at some moment (say by convention, the start of the date or time expressed perhaps down to nanoseconds); or it is a range, from the start of the date or time (expressed in terms of the shortest unit used in calculation, unless you want to fiddle with going down to Planck times to allow for future increase in precision) up to but not including the start of the next increment at the stated precision (for a time in seconds with a minimum resolution of nanoseconds, the billion nanoseconds starting at the beginning of the second).

So,an hour range starting at a half hour boundary might fall entirely within a day, entirely outside that day, or half in that day and half before or after it.

All of that neglects time zones, or assumes they're at most recorded for local convenience and/or historical purposes but converted for calculation into UTC or the like.

So, for instants, either they're equal or they're not; but for ranges, they could be equal, or a smaller range entirely within a larger, or partly overlapping, or disjoint. Pick whichever model (instant or range) works best, and be consistent thereafter, and at least the surprises shouldn't be too surprising.

The specific question is strangely put. Why 12:00?

In the generic case where one has conflicting data types to work with, including all sorts of lost precision, the boring answer is that it is context dependent.

Direct user interfacing applications should follow the principle of least surprise, which for an interval search would be to always treat the value as inclusive when in doubt.

The root of most date/time problems stems from the fact that we use/have insufficient vocabulary to define what we actually want.

For example, "time" can be used as "time of day" or "duration". In my own work we have to measure "from this time, to that time" and yhd result should be duration. [1]

We also use the phrase Time when we mean Timestamp (a date /time combination). And we use "local timezone" implicitly almost everywhere, where we should be storing everything as UTC and then displaying as desired. (this would make comparisons trivial.)

Daylight savings is an abomination since it means time is not contiguous, and the sooner it goes away the better. This is somewhat solved by using UTC when time-stamping.

Overall I prefer storing everything as UTC, then displaying that as preferred by each user (ie as their local time).

[1] should be, but is instead stored as a time field for historical design reasons.

This site finds the midpoint between date/times as well as the difference between them. Also, time units can be added or subtracted from one date/time to get another. All of this is done through the JavaScript Date object which leaves to trace of the original time zone in arriving at the offset in milliseconds since 1970. If doing the difference between dates in days, round the difference to the nearest day to avoid problems with daylight savings time not being in effect for one of them. https://wjporter.com/misc/MidPoint.htm

Type error.

A `date` is a time interval (from midnight to midnight in a given time zone). One could argue that `time` values in computer programs also cover a span, even if it is infinitesimally small for most purposes.

The correct operation to compare intervals of varying lengths is not equality, it is either containment or overlap.

What date/time mistakes did UNESCO make that the Ruby documentation is referring to?

Isn't this just a matter of being clear of inclusive or exclusive ranges? Perhaps this is merely a provocation that the general perception of exclusivity or inclusivity rests in the casual programmer's perception (rather than the specification of which I confess ignorance)?

Or is the provocation in measuring the contour of a coast? Can it be measured more precisely (surely it can)? In which case is the provocation one of mere precision?

Or is the provocation one which asks how many fairies may fit on the head of a pin?

A list of date/time/calendar quirks and oddities:

https://yourcalendricalfallacyis.com/

No. No. No. Dates are to Integers as Date+Time is to Floating Point. You (we) -want- to compare them, but you need something akin to a language level specification for these. Otherwise, it is all what you are trying to do/what problem you are solving. Excel (where I suspect 60% or more of this nonsense lives) at least has a clear definition (both are floats, so you can do the comparison). Otherwise, "Down the Wat-hole" for what to expect.

I never understood why programmers have such a hard time with time. There are two and only two pieces of information you need to work with time: seconds past the epoch and location. Everything else is applying legal and social context to the actual data.

Here both of those pieces of information are missing on all three questions, so no answers can be given. Any answer to these questions makes assumptions or deductions about those pieces of information.

My (naive) answer would be to localize any DateTime to account for any changes in Date due to timezone, strip the time and compare the Dates. I'd reckon a Date means any time in that date and is of lower specificity than a DateTime, rather than midnight on that Date. Is there any obvious issue with this in a situation where you might be (inadvisably) comparing a Date to a DateTime?

I was quick to say that a date equals a datetime at 0 hours and 0 minutes, but after reading and thinkiong about it, I agree that the question is wrong, but a little rephrasing can probably help solve the problem at hand:

- Does this datetime equal this date? No, no it doesn't and never will.

- Does this datetime fall on this date? Yes, it might.

- Is this date, the date of this datetime? Yes, it might be.

"It depends"

First, are we talking about what _is_ or what _should be_?

Second, what is the business logic behind the choice of Date versus Time to represent data (a data point or a segment) in the timeline? And is that logic consistent with the original design, and therefore with how the data have been collected?

There is more, of course. Lots more, some touched upon in the article and comments.

If I were to design a new programming language then x > y or x == y when x is the float 1.2 and y the integer 1 would throw an exception. Mathematically that is wrong because obviously 1.2 > 1, but in my experience, trying to compare values of different types leads to much pain and many annoying bugs.

Isn’t DateTime fairly consistently just a singular moment it time, either absolute (TZ specified) or floating and therefore locale dependent?

Pretty sure these are the two cases which the python stdlib datetime covers.

It’s not no, it’s undecidable. They are different types so there’s no equality operator defined.

I would argue that both dates and datetimes dimensionally signify ranges of time with different lengths. A date being a range of ~24 hours and a time being a range of a minute/second/microsecond depending on the definition. It's rarely truly meant to signify an instantaneous point in time.

Looked at as ranges, equality can be defined as whether they overlap, and that works for most use cases but not all.

The only right model of dates and times is Java Time API. This is an example of great API design that everyone must be familiar with. For example, they have different representations of time there that are context-specific. Time is not always an instant and not always require a time zone: when you specify opening hours of a shop, they are defined as a LocalTime. ZonedDateTime is not the same as Instant in the context of calculations. Adding a day to instant always adds 24 hours. Adding a day to the time in the time zone observing DST may result in adding 23 or 25 hours. Time is not hard, it’s just you always need enough context to pick the right representation.

No. Date are the msb’s of time.

I wonder what kind of horrible design decision led to this in the first place.

I’d argue though, that without context, a date has the time 00:00 in your local time zone.

    let date1 = Date("06/05/2022")
    let date2 = DateTime("06/05/2022 20:05 UTC")
    if date2 > date1 {
        // SHOULD NOT COMPILE
    }

    let date3 = Date("06/05/2022").withTime("05:00 UTC")
    if date2 > date3 {
        // DOES COMPILE
    }

I'm not sure I follow.

If I define a date MM/DD/YYYY as equal to the time MM/DD/YYYY 00:00 GMT, which of the equality properties am I lacking?

Similarly, if I use that definition of equality to convert the date into a time, and then compare it, can't I get '>' and '<'?

This is why experienced programmers will choose closed-open intervals by default -- they sidestep many of these pitfalls. For your example, you would do

  ... and date >= '2020-02-13' and date < '2020-02-14' -- notice: >= and <

Since this also applies to real numbers, maybe it is easier to keep in mind with an example in that realm. You would never check for a real number in a given range with:

  ... and X >= 3.0 and X <= 3.9 // or 3.99, or 3.999999

but instead, of course, you would do:

  ... and X >= 3.0 and X < 4

And the scales just fell from my eyes. I'm guilty of this in the system I develop at work. Whenever I want to show the user all widgets that were made on the date they selected, I will just do something like

    DATE(widget_made) = :user_date

Which does what I want and avoids the problem you brought up. But sometimes I have some more complicated logic that falls down if I try to compare datewise. So I manually create the DateTime ranges, and use 23:59:59 as the end of the range. So something like

    widget_made BETWEEN :user_date '00:00:00' AND :user_date '23:59:59'

Anything that came off the line at 23:59:59.3421 will be excluded. Fortunately for me, I don't think that has ever actually happened. But now I know to be on the lookout, and use proper date-handling tools to ensure correctness.

Right. The best way to think about it is that any implicit values which are required to univocally define a timestampz should be considered as NULL. So 2022-02-12 should mean “some time during such day in some time zone”.

If you really want to blow their minds tell them only some days are 24 hours long.