D3.js - get number of days for a certain month

Question

I'm using D3.js (v3) to plot a time-series graph and I'm trying to figure out the amount of ticks I need for a given month (ie, days). From what I can make of the documentation, d3.time-month should return something from 28 to 31 days, which is what I want, but for some reason I'm obviously missing, I can't seem to get the number I need.

Can anyone help?

This is what I've got so far:

console.log(date); // Fri Jul 01 2016 00:00:00 GMT+0100 (WEST)

monthDays = d3.time.month(date);

console.log(currentMonth+" has "+monthDays+" days."); // July has Fri Jul 01 2016 00:00:00 GMT+0100 (WEST) days.

See http://stackoverflow.com/q/1184334/6210816 – Tim B Aug 01 '16 at 14:34 — Tim B, Aug 01 '16 at 14:34

score 1 · Accepted Answer · edited May 23 '17 at 11:44

You're misreading the documentation on intervals : the number of days in the intervals used by d3.time.month have a length between 28 and 31 but the functions are defined as

interval(date)

Alias for interval.floor(date). [...]

and

interval.floor(date)

Rounds down the specified date, returning the latest time interval before or equal to date. [...]

Basically, d3.time.month(date) will return the first day of the month at midnight, not the number of days in that month.

How to get the number of days then? As far as I can tell, D3 does not expose a way to get the length of the month for a given date. You could of course get the range of days for a given month and extract its length:

var date = new Date(2016, 01, 02); // 2016-02-02
console.log(
    d3.time.days(d3.time.month(date), d3.time.month.ceil(date)).length
)

or probably more efficiently use plain JS like in this answer https://stackoverflow.com/a/1185804/1071630 :

var date = new Date(2016, 01, 02); // 2016-02-02
console.log(
    new Date(date.getFullYear(), date.getMonth()+1, 0).getDate()
)

var date = new Date(2016, 01, 02);

console.log(
  d3.time.days(d3.time.month(date), d3.time.month.ceil(date)).length
)

console.log(
 new Date(date.getFullYear(), date.getMonth()+1, 0).getDate()
)

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>

If you came here looking for a quick way to find the number of days between two d3.Date objects, check out [D3-Time](https://github.com/d3/d3-time/blob/master/README.md#interval_count), specifically the `interval.count()` method. — Austin A, Jan 26 '18 at 05:40

D3.js - get number of days for a certain month

1 Answers1