About Philip Chimento
A couple of tough questions for all of you:1. Is the date 2022-06-01 equal to the time 2022-06-01 ?2. Is the date 2022-06-01 between the time 2022-06-01 and the time 2022-12-31 ?3. Is the time 2022-06-01 after the date 2022-06-01 ?
I’ve been involved for two years and counting 1 in the design of Temporal, an enhancement for the JavaScript language that adds modern facilities for handling dates and times. One of the principles of Temporal that was established long before I got involved, is that we should use different objects to represent different concepts. For example, if you want to represent a calendar date that’s not associated with any specific time of day, you use a class that doesn’t require you to make up a bogus time of day. 2 Each class has a definition for equality, comparison, and other operations that are appropriate to the concept it represents, and you get to specify which one is appropriate for your use case by your choice of which one you use. In other, more jargony, words, Temporal offers different data types with different semantics. 3
For me these questions all boil down to, when we consider a textual representation like 2022-06-01 , what concept does it represent? I would say that each of these strings can represent more than one concept, and to get a good answer, you need to specify which concept you are talking about.
So, my answers to the three questions are “it depends”, “no but maybe yes”, and “it depends.” I’ll walk through why I think this, and how I would solve it with Temporal, for each question.
You can follow along or try out your own answers by going the Temporal documentation page, and opening your browser console. That will give you an environment 
where you can try these examples and experiment for yourself.
Question 1
As I mentioned above, Temporal has different data types with different semantics. In the case of this question, what the question refers to as a “time” we call a “date-time” in Temporal 4 , and the “date” is still a date. The specific types we’d use are PlainDateTime and PlainDate , respectively. PlainDate is a calendar date that doesn’t have a time associated with it: a single square on a wall calendar. PlainDateTime is a calendar date with a wall-clock time. In both cases, “plain” refers to not having a time zone attached, so we know we’re not dealing with any 23-hour or 25-hour or even more unusual day lengths.
The reason I say that the answer depends, is that you simply can’t say whether a date is equal to a date-time. They are two different concepts, so the answer is not well-defined. If you want to do that, you have to convert one to the other so that you either compare two dates, or two date-times, each with their accompanying definition of equality.
You do this in Temporal by choosing the type of object to create, PlainDate or PlainDateTime , and the resulting object’s equals() method will do the right thing:
I think either of PlainDate or PlainDateTime semantics could be valid based on your application, so it seems important that both are within reach of the programmer. I will say that I don’t expect PlainDateTime will get used very often in practice. 5 But I can think of a use case for either one of these:
- If you have a list of PlainDateTime events to present to a user, and you want to filter them by date. Let’s say we have data from a pedometer, where we care about what local time it was in the user’s time zone when they got their exercise, and the user has asked to see all the exercise they got yesterday. In this case I’d use date semantics: convert the PlainDateTime data to PlainDate data.
