Let's say I have 3 variables like this:

var Month = 8; // in reality, it's a parameter provided by some user input
var Year = 2011; // same here
var FirstDay = new Date(Year, Month, 1);

Now I want to have the value of the day before the first day of the month in a variable. I'm doing this:

var LastDayPrevMonth = (FirstDay.getDate() - 1);

It's not working as planned. What the right of doing it?

Thanks.

Let's say I have 3 variables like this:

var Month = 8; // in reality, it's a parameter provided by some user input
var Year = 2011; // same here
var FirstDay = new Date(Year, Month, 1);

Now I want to have the value of the day before the first day of the month in a variable. I'm doing this:

var LastDayPrevMonth = (FirstDay.getDate() - 1);

It's not working as planned. What the right of doing it?

Thanks.

• javascript

5 Answers 5

var LastDayPrevMonth = new Date(Year, Month, 0).getDate();

var LastDayPrevMonth = new Date(FirstDay);
LastDayPrevMonth.setHours(FirstDay.getHours()-24);

var FirstDay = new Date(Year, Month, 1);
var lastMonth = new Date(FirstDay);
lastMonth.setDate(-1);
alert(lastMonth);

And remember that 8 is Sept, not Aug in JavaScript. :)

If you need to calculate this based on today's date ( you want the last day of last month ), the following should help.

If you only care about the month/day/year, this is the simplest and fastest that I can think of:

var d = new Date(); d.setDate(0);
console.log(d);

If you want midnight of last day of the previous month, then:

var d = new Date(); d.setDate(0); d.setHours(0,0,0,0);
console.log(d);

If you want to know the last day of the previous month, based on provided year/month:

var year = 2016, month = 11;
var d = new Date(year, (month - 1)); d.setDate(0); d.setHours(0,0,0,0);
console.log(d);

After running any of the above, to get the YYYY-M-D format:

var str = d.getFullYear() + "-" + (d.getMonth() + 1) + "-" + d.getDate();
console.log(str);

To see additional methods available, and to see what they do, you can read the docs.

Create a new Date object and pass it the other date coerced to the number of milliseconds since the Unix epoch and then minus a whole day (in milliseconds).

var LastDayPrevMonth = new Date(FirstDay - 864e5);

Example:

var Month = 8; // in reality, it's a parameter provided by some user input
var Year = 2011; // same here
var FirstDay = new Date(Year, Month, 1);

var LastDayPrevMonth = new Date(FirstDay - 864e5);

document.body.innerHTML = LastDayPrevMonth;