Formatting a date is giving me a different one from the passed one

user_12506687 0
Viewed 64

I need to get the first and last day of the current month, this works perfectly but when I format the date to looks like this yyyy/mm/dd it changes from the one I passed

For example, if a passed the 1 May 2020 when I format the date returns me the 30 April 2020

var startID = document.getElementById('start');
var datenow = new Date();
var firstday = new Date(datenow.getFullYear(), datenow.getMonth(), 1);
firstday = firstday.toISOString().slice(0, 10);
startID.value = firstday;
<input type="date" name="start" class="form-control" id="start" required autofocus>
<label for="start">Start Date</label>

This is how I get the first day of the current month:

var datenow = new Date();
var firstday = new Date(datenow.getFullYear(),datenow.getMonth(), 1);

This is how I format the date to looks like yyyy/mm/dd in this part is where I get another wrong date from the passed one:

firstday = firstday.toISOString().slice(0,10);
javascript
4 Answers

The root cause of the problem is in the use of getMonth() method to obtain the month number.

The getMonth() method returns integers from 0 to 11 (January = 0, ...Dec = 11).

The problem can be fixed by adding 1 to getMonth().

Here is a working example to show current date, first day and last day:

var now = new Date()
console.log("Today = " + now)

let month = (now.getMonth() + 1)
let year = now.getFullYear()

firstDay = ("1/" + month + "/" + year)
lastDay = ((new Date(year, month, 0)).getDate() + "/" + month + "/" + year)

console.log("First day = " + firstDay)
console.log("Last day = " + lastDay)

Output:

Today = Sat May 30 2020 20:54:56 GMT+0100 (British Summer Time)
First day = 1/5/2020
Last day = 31/5/2020

More information:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getMonth

user_65711650

Timezones are tricky! Any given timestamp could look like a different date depending on your timezone. ISO strings ignore timezone. So for starters, you could just do new Date().toString().

user_54782840

That happens because firstday.toISOString() returns date in UTC timezone so if you're located somewhere else you can expect the shifts in dates.

Unfortunately there is no go-to solution if you're not using something like moment.js (and it does not seem to be appropriate for such a simple task). I'd recommend you something like:

const [day, month, year] = firstday.toLocaleDateString('en-US').split('.');
firstday = [year, month, day].join('/')
user_60128580

use moment.js to properly format dates and perform date related operations. it makes thing fairly easier to implement.

user_87639570
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