Develop Reference

Modify "Round to nearest interval" function so that it works with intervals above 1 hour

Consider this function that rounds down to the nearest interval:
function roundToNearest(value, interval) {
return Math.floor(value/interval) * interval;
}
Date.now()
> Thu Sep 01 2022 05:38:11 GMT+0300 (Eastern European Summer Time)
Now run it with 5 minutes:
new Date(roundToNearest(Date.now(), 1000*60*5))
> Thu Sep 01 2022 05:35:00 GMT+0300 (Eastern European Summer Time)
15 minutes:
new Date(roundToNearest(Date.now(), 1000*60*15))
> Thu Sep 01 2022 05:30:00 GMT+0300 (Eastern European Summer Time)
1 hour:
new Date(roundToNearest(Date.now(), 1000*60*60*1))
> Thu Sep 01 2022 05:00:00 GMT+0300 (Eastern European Summer Time)
2 hours:
new Date(roundToNearest(Date.now(), 1000*60*60*2))
>Thu Sep 01 2022 03:00:00 GMT+0300 (Eastern European Summer Time) (should be 04:00:00)
Intervals 1 hour and below return the expected results, but 2 hours (and any interval above 2 hours) does not (04:00:00 was expected for 2 hours, for example). How can I modify it so that it works on intervals above 1 hour?

I think what you need here is to round the time since midnight. That is, if the time is 05:30 with 2 hour rounding you want 04:00, or for 6 hour rounding you want 00:00.
We get the interval since local midnight, round to the desired number, then add to midnight. We'll create a few helper functions getMidnight() and getTimeSinceMidnight().
We'll then combine to create a roundSinceMidnight() function.
function roundSinceMidnight(date, interval) {
return roundToNearest(getTimeSinceMidnight(date), interval) + getMidnight(date);
}
function roundToNearest(value, interval) {
return Math.floor(value/interval) * interval;
}
function getMidnight(date) {
const d = new Date(date); // date could be a Date object or ms since 1970...
return new Date(d.getFullYear(), d.getMonth(), d.getDate()).getTime();
}
function getTimeSinceMidnight(date) {
return date - getMidnight(date);
}
console.log('1 hour: ', new Date(roundSinceMidnight(Date.now(), 1000*60*60*1)).toTimeString())
console.log('2 hours:', new Date(roundSinceMidnight(Date.now(), 1000*60*60*2)).toTimeString())
console.log('4 hours:', new Date(roundSinceMidnight(Date.now(), 1000*60*60*4)).toTimeString())
console.log('6 hours:', new Date(roundSinceMidnight(Date.now(), 1000*60*60*6)).toTimeString())
.as-console-wrapper { max-height: 100% !important; }

You may pass over for the time difference in your timezone. Let's subtract +0300 from 03:00:00. I don't prove it correct, but the following might work for you:
new Date(roundToNearest(Date.now() + 1000*60*60*3, 1000*60*60*2) - 1000*60*60*3)

Why is getDate() a month off?

I'm trying to iterate over objects that have a date within a specific time frame. These time frames are days in the past week bounded by startDate and endDate.
I have a for loop that sets the startDate to the beginning of the day, and endDate will be the end of the day.
let startDate = new Date(); // startDate: Start bound of average execution time calculation
let endDate = new Date(); // endDate: End bound of average execution time calculation
for (let i = 0; i < 7; i++) {
// i is used to decrement the startDate to i days from today
// set startDate to beginning of the day
startDate = new Date();
startDate.setDate(startDate.getDate() - i);
startDate.setHours(0, 0, 0, 0);
console.log(i + ": " + startDate);
// set endDate to end of the day
endDate.setDate(startDate.getDate());
endDate.setHours(23, 59, 59, 999);
console.log(i + ": " + endDate);
However, even though I'm setting the endDate to the startDate, the console tells me that the endDate is one month off. Why is this happening?
The console shows this:
0: Sat Aug 01 2020 00:00:00 GMT-0700 (Mountain Standard Time)
0: Sat Aug 01 2020 23:59:59 GMT-0700 (Mountain Standard Time)
1: Fri Jul 31 2020 00:00:00 GMT-0700 (Mountain Standard Time)
1: Mon Aug 31 2020 23:59:59 GMT-0700 (Mountain Standard Time)
2: Thu Jul 30 2020 00:00:00 GMT-0700 (Mountain Standard Time)
2: Sun Aug 30 2020 23:59:59 GMT-0700 (Mountain Standard Time)
3: Wed Jul 29 2020 00:00:00 GMT-0700 (Mountain Standard Time)
3: Sat Aug 29 2020 23:59:59 GMT-0700 (Mountain Standard Time)
4: Tue Jul 28 2020 00:00:00 GMT-0700 (Mountain Standard Time)
4: Fri Aug 28 2020 23:59:59 GMT-0700 (Mountain Standard Time)
5: Mon Jul 27 2020 00:00:00 GMT-0700 (Mountain Standard Time)
5: Thu Aug 27 2020 23:59:59 GMT-0700 (Mountain Standard Time)

Think like arrays, where the first element is 0. The same is true here. The months of the year will be 0 - 11, rather than 1 - 12. The getMonth() method returns the month in the specified date according to local time, as a zero-based value (where zero indicates the first month of the year).
Also, keep this in mind: With setDate() the expected values are 1-31, but other values are allowed: 0 will result in the last day of the previous month. -1 will result in the day before the last day of the previous month.

So, I ended up just fixing it by not referring to startDate at all. I'm not sure why I can't call getDate() without getting the wrong date, but for now, I'll just set endDate by repeating whatever I did to startDate.
// set startDate to beginning of the day
startDate = new Date();
startDate.setDate(startDate.getDate() - i);
startDate.setHours(0, 0, 0, 0);
console.log(i + ": " + startDate);
// set endDate to end of the day
endDate = new Date();
endDate.setDate(endDate.getDate() - i);
endDate.setHours(23, 59, 59, 59);
console.log(i + ": " + endDate);

The reason it happens is because you are setting the date but not the month, so when the loop crosses a month boundary, the start goes to the previous month but end stays in the current month.
When run on 1 Aug, in the first iteration both start and end are set to 1 Aug.
In the next iteration, 1 is subtracted from the start so it's 31 July, but then just the date is set for end, so it goes to 31 Aug, and so on…
A reworking of the code:
// Start at random time on 1 Aug
let d = new Date(2020,7,1,15,23,51,3);
for (let start, end, i=0; i<7; i++) {
start = new Date(d.setHours(0,0,0,0));
console.log(i + ' : ' + start.toString());
end = new Date(d.setHours(23,59,59,999));
console.log(i + ' : ' + end.toString());
d.setDate(d.getDate() - 1);
}

javascript - date difference should be zero but it is 18 hours

This one has stumped me. It should be so simple I would think. I am doing some very simple date subtraction in Javascript. I am subtracting the same dates and I would think it would give zero hours, but it gives 18 hours.
let inDate = new Date('Tue Aug 27 2019 00:00:00 GMT-0500 (Central Daylight Time)').getTime();
let outDate = new Date('Tue Aug 27 2019 00:00:00 GMT-0500 (Central Daylight Time)').getTime();
document.getElementById('date').innerHTML = new Date(outDate - inDate);
<div id='date'>
</div>
In case it produces different results based on where you are, the result I am getting is this:
Wed Dec 31 1969 18:00:00 GMT-0600 (Central Standard Time)

This is due to your timezone. If you convert to GMT String before print it the time will be correct. (Jan 01, 1969 00:00:00)
new Date(outDate - inDate).toGMTString()
You should see the correct date.
let inDate = new Date('Tue Aug 27 2019 00:00:00 GMT-0500 (Central Daylight Time)').getTime()
let outDate = new Date('Tue Aug 27 2019 00:00:00 GMT-0500 (Central Daylight Time)').getTime()
console.log(new Date(inDate - outDate).toGMTString())

Difference between 2 dates, should work, but it isn't

i have got the following code which should simply tell me difference between 2 months, however all that it is returning is 1 and I can't figure it out!
function parseDate(str) {
function pad(s) { return (s < 10) ? '0' + s : s; }
var d = new Date(str);
return d;
}
Array.prototype.monthDifference = function() {
var months = this[1].getMonth() - this[0].getMonth() + (12 * (this[1].getFullYear() - this[0].getFullYear()));
if(this[1].getDate() < this[0].getDate()){
months--;
}
return months;
};
console.log([parseDate('01/01/2017'), parseDate('02/04/2017')].monthDifference());
Edit
Okay, see updated code below:
Array.prototype.monthDifference = function() {
console.log((this[1].getMonth()+1) - (this[0].getMonth()+1));
var months = (this[1].getMonth()+1) - (this[0].getMonth()+1) + (12 * (this[1].getFullYear() - this[0].getFullYear()));
if(this[1].getDate() < this[0].getDate()){
months--;
}
return (months > 1) ? 0 : months;
};
[pubDate, new Date()].monthDifference();
And now the output, how is one of the numbers negative and the other positive!? And comparing against today and dates in the past...
1
Sat Apr 27 1907 00:00:00 GMT+0100 (BST) Wed May 28 1902 00:00:00 GMT+0100 (BST)
-10
Wed Mar 26 1930 00:00:00 GMT+0000 (GMT) Wed May 28 1902 00:00:00 GMT+0100 (BST)
-10
Tue Mar 26 1929 00:00:00 GMT+0000 (GMT) Wed May 28 1902 00:00:00 GMT+0100 (BST)
-10
Tue Mar 26 1929 00:00:00 GMT+0000 (GMT) Wed May 28 1902 00:00:00 GMT+0100 (BST)
-1
Tue Jun 24 1913 00:00:00 GMT+0100 (BST) Wed May 28 1902 00:00:00 GMT+0100 (BST)

What about this ?
It gives days between two date.
Array.prototype.monthDifference = function() {
var b = this[0].getTime();
var x = this[1].getTime();
var y = x-b;
return Math.floor(y / (24*60*60*1000));
};
var a = [];
a.push(parseDate('01/01/2016'));
a.push(parseDate('02/04/2017'));
console.log(a.monthDifference());

The JavaScript Date constructor doesn't parse strings in UK format(dd/mm/yyyy).
You can split the date string and and then pass it into Date constructor.
Working fiddle: Date foramte fiddle
function formateDateToUK(dateString){
var splitDate = dateString.split('/'),
day = splitDate[0],
month = splitDate[1] - 1, //Javascript months are 0-11
year = splitDate[2],
formatedDate = new Date(year, month, day);
return formatedDate;
}

you functions returns '1', since it is the correct result :)
try:
console.log([parseDate('01/01/2017'), parseDate('07/01/2017')].monthDifference());
and it returns '6'... which is correct.
Note: 'new Date(str)' expects "MM/dd/yyyy" not "dd/MM/yyyy".
Hope this helps

31 days in February in Date object

This code should log all days for given month:
var date = new Date(2012,2,1);
var thisMonth = date.getMonth();
while(date.getMonth()==thisMonth) { // 31 steps ???
console.log(date.getMonth(),date.getDate());
date.setDate(date.getDate()+1);
}
It works well for every month but February. Any ideas where is the catch?

Note the month parameter is 0-indexed, so your code is about March not February.
The doc:
month
Integer value representing the month, beginning with 0 for January to
11 for December.

Use new Date(2012,1,1); month is zero-based ;-)

This is pretty interesting:
new Date('2014-02-28'); // Fri Feb 28 2014 01:00:00 GMT+0100
new Date('2014-02-29'); // Sat Mar 01 2014 01:00:00 GMT+0100
new Date('2014-02-30'); // Sun Mar 02 2014 01:00:00 GMT+0100
new Date('2014-02-31'); // Mon Mar 03 2014 01:00:00 GMT+0100
new Date('2014-02-32'); // Invalid Date