How to do date rounding in Javascript?

Question

I have an array of dates that I want to round to various frequencies - weeks, months, quarters, and years. The equivalent of rounding numbers but for dates.

Here is a sample array:

const dates = [
  "1/4/2020",
  "2/5/2021",
  "8/15/2021",
];

Rounding by month would yield:

[
  "1/31/2020",
  "2/28/2021",
  "8/31/2021",
];

Nearest year:

[
  "12/31/2020",
  "12/31/2021",
  "12/31/2021",
];

And so on. Is there a Javascript library that allows for such rounding? If not, is there a simple function that would cover the above frequencies? I can write a custom function for each use case but that'll likely be inefficient.

unclear how you get nearest year values.... How does February differ from January?? — epascarello, Aug 24 '21 at 17:02
I don't believe there is a JS library for this. must be coded. What did you try to do? — Mister Jojo, Aug 24 '21 at 17:15
Asking for libraries is specifically off-topic on Stack Overflow. I'd remove that question if I were you. — Heretic Monkey, Aug 24 '21 at 17:27
Also, please [edit] your question to explain the rounding rules, and what you have tried so far, if you'd like help with that. — Heretic Monkey, Aug 24 '21 at 17:28
Apologies everyone - I'll do some more work and ask a better question — mmz, Aug 24 '21 at 18:21

some coder guy · Accepted Answer · 2021-08-24T17:41:52.750

1

Assuming there is a typo in your first array of dates and "2/5/2020" was meant to be "2/5/2021" (which would explain your desired results), this code should work:

function roundMonth(dateString) {
  var date = new Date(Date.parse(dateString));
  var year = date.getFullYear();
  var month = date.getMonth();
  var date = new Date(year, month + 1, 0); 
    // day 0 yields the last day of the previous month, 
    // so we add 1 to the month value
  var year = date.getFullYear();
  var month = date.getMonth();
  var day = date.getDate();
  var date = [month + 1,day,year].join("/");
    // for string output we add 1 to month since Jan is month 0
  return date;
}

function roundYear(dateString) {
  var date = new Date(Date.parse(dateString));
  var year = date.getFullYear();
  var date = [12,31,year].join("/");
  return date;
}

edited Aug 24 '21 at 17:41

answered Aug 24 '21 at 17:23

some coder guy

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Using `new Date(string)` with some formats is risky and browser-, OS-, and locale-specific. See [Why does Date.parse give incorrect results?](https://stackoverflow.com/questions/2587345/why-does-date-parse-give-incorrect-results) – Heretic Monkey Aug 24 '21 at 17:30
@HereticMonkey good to know, thanks. – some coder guy Aug 24 '21 at 17:34
1

`new Date(Date.parse(dateString))` will produce an identical result to `new Date(dateString)`, so the call to *Date.parse* is redundant. But you shouldn't be parsing unsupported formats with the built–in parser anyway, per @HereticMonkey's comment. – RobG Aug 24 '21 at 22:19

How to do date rounding in Javascript?

1 Answers1