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JavaScript setHours() and getHours() [duplicate]

I am trying to subtract hours from a given date time string using javascript.
My code is like:
var cbTime = new Date();
cbTime = selectedTime.setHours(-5.5);
Where selectedTime is the given time (time that i pass as parameter).
So suppose selectedTime is Tue Sep 16 19:15:16 UTC+0530 2014
Ans I get is : 1410875116995
I want answer in datetime format.
Am I doing something wrong here? Or there is some other solution?

The reason is that setHours(), setMinutes(), etc, take an Integer as a parameter. From the docs:
...
The setMinutes() method sets the minutes for a specified date
according to local time.
...
Parameters:
An integer between 0 and 59, representing the minutes.
So, you could do this:
var selectedTime = new Date(),
cbTime = new Date();
cbTime.setHours(selectedTime.getHours() - 5);
cbTime.setMinutes(selectedTime.getMinutes() - 30);
document.write('cbTime: ' + cbTime);
document.write('<br>');
document.write('selectedTime: ' + selectedTime);

Well first off setting the hours to -5.5 is nonsensical, the code will truncate to an integer (-5) and then take that as "five hours before midnight", which is 7PM yesterday.
Second, setHours (and other functions like it) modify the Date object (try console.log(cbTime)) and return the timestamp (number of milliseconds since the epoch).
You should not rely on the output format of the browser converting the Date object to a string for you, and should instead use get*() functions to format it yourself.

According to this:
http://www.w3schools.com/jsref/jsref_sethours.asp
You'll get "Milliseconds between the date object and midnight January 1 1970" as a return value of setHours.
Perhaps you're looking for this:
http://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_sethours3
Edit:
If you want to subtract 5.5 hours, first you have to subtract 5 hours, then 30 minutes. Optionally you can convert 5.5 hours to 330 minutes and subtract them like this:
var d = new Date();
d.setMinutes(d.getMinutes() - 330);
document.getElementById("demo").innerHTML = d;

Use:
var cbTime = new Date();
cbTime.setHours(cbTime.getHours() - 5.5)
cbTime.toLocaleString();

try this:
var cbTime = new Date();
cbTime.setHours(cbTime.getHours() - 5.5)
cbTime.toLocaleString();

How to programmatically determine the prior time period based upon unix time stamps in Javascript?

Essentially I have two unix timestamps, representing the first and last days of a given month. Is it possible programmatically determine the timestamps for the first and last of the previous month?
For example, I have the following two timestamps:
1467331201 --> July 1, 2016
1469923201 --> July 31, 2016
Essentially, can I manipulate these two numbers in a consistent way in order to the unix time (or Date object) for June 1, 2016 and June 30, 2016, respectively? Problem that I'm running into is that you cannot simply subtract a given amount because the amount of days in a month is variable.

You could use this function:
function getPreviousMonthRange(unixTime) {
var dt = new Date(unixTime * 1000);
dt.setUTCDate(0); // flips to the last day of previous month
var unixLast = dt.getTime();
dt.setUTCDate(1); // back to the first day of that same month
var unixFirst = dt.getTime();
return [unixFirst / 1000, unixLast / 1000];
}
// given first and last date (only one is really needed)
var unixTimeFirst = 1467331201;
var unixTimeLast = 1469923201;
// get previous month's first & last date
var [first, last] = getPreviousMonthRange(unixTimeFirst);
// output
console.log('previous month first day: ', first, new Date(first*1000));
console.log('previous month last day: ', last, new Date(last*1000));

Take a look at the following example:
// Specify a timestamp
var timestamp = 1467331201;
// Create a date object for the time stamp, the object works with milliseconds so multiply by 1000
var date = new Date(timestamp * 1000);
// Set the date to the previous month, on the first day
date.setUTCMonth(date.getUTCMonth() - 1, 1);
// Explicitly set the time to 00:00:00
date.setUTCHours(0, 0, 0);
// Get the timestamp for the first day
var beginTimestamp = date.getTime() / 1000;
// Increase the month by one, and set the date to the last day of the previous month
date.setUTCMonth(date.getUTCMonth() + 1, 0);
// Explicitly set the time to 23:59:59
date.setUTCHours(23, 59, 59);
// Get the timestamp for the last day
var endTimestamp = date.getTime() / 1000;
// Print the results
console.log('Timestamps for previous month: ');
console.log('Begin timestamp: ' + beginTimestamp);
console.log('End timestamp: ' + endTimestamp);
A timestamp must be specified in the variable on the top, this might be one of the two timestamps you suggested in your question, anywhere in a month.
This code then calculates the begin and end timestamp for the previous month as you've requested, and prints the results to the console.
Please note, that in this example the begin timestamp uses 00:00:00 as time, and the end timestamp uses 23:59:59 as time (the last second of that day). This can be configured the way you'd prefer.
In this case, we're working with the ...UTC... Date functions, because a Unix timestamp is in UTC time, not in the timezone the user is in.
The statement date.setMonth(date.getMonth() + 1, 0); is used to select the last day in the month. The next month is selected first, but because the day is set to 0 (and not 1) one day is subtracted giving you the preferred result. This is described here.

You can consider using Moment.js. I'm sure this is not exactly how you'd end up handling it but see below for an example of some helpful methods.
var lastDayOfJuly = moment(1469923201);
var firstDayOfJuly = lastDayOfJuly.startOf('month');
var lastDayOfJune = firstDayOfJuly.subtract(1, 'day');
var firstDayOfJune = lastDayOfJune.startOf('month");
Moment.js

How to subtract 2 times with moment.js, then subtract some minutes

I need to subtract 2 times with moment.js (get the difference), and then with that result, subtract some additional minutes (simple int). It's for calculating timesheets. A few examples:
Example #1:
Start time: 10:00 AM (represented in js as "10:00")
End time: 2:00 PM (represented in js as "14:00")
Lunch: 30 minutes ("30")
Expected result: "3:30" (10am - 2pm is 4 hours, minus 30 minutes for lunch = 3hrs 30 mins -- and I need it output as "3:30")
Example #2:
Start time: 6:15 AM (represented in js as "6:15")
End time: 4:45 PM (represented in js as "16:45")
Lunch: 0 minutes ("0")
Expected result: "10:30"
I know moment.js can do this but I'm struggling to get expected results. I've been trying this:
function getTimeInterval(startTime, endTime){
return moment(moment(startTime,"hh:mm").diff(moment(endTime,"hh:mm"))).format("hh:mm");
}
The formatting seems right, but I'm getting incorrect values. For example, the result returned for my example #2 is "6:30" instead of "10:30" And then how do I subtract off int minutes for lunch?
Any help is much appreciated.

// parse time using 24-hour clock and use UTC to prevent DST issues
var start = moment.utc(startTime, "HH:mm");
var end = moment.utc(endTime, "HH:mm");
// account for crossing over to midnight the next day
if (end.isBefore(start)) end.add(1, 'day');
// calculate the duration
var d = moment.duration(end.diff(start));
// subtract the lunch break
d.subtract(30, 'minutes');
// format a string result
var s = moment.utc(+d).format('H:mm');
Pay close attention to the casing of the formats. You were using hh which is for a 12-hour clock.
See also: Get the time difference between two datetimes

You can use the diff method to calculate the difference between two dates and the subtract method to subtract time. In your case:
function getTimeInterval(startTime, endTime, lunchTime){
var start = moment(startTime, "HH:mm");
var end = moment(endTime, "HH:mm");
var minutes = end.diff(start, 'minutes');
var interval = moment().hour(0).minute(minutes);
interval.subtract(lunchTime, 'minutes');
return interval.format("HH:mm");
}

javascript wrong date / days calculation

I need to calculate the number of nights between 2 dates, it works but it's very odd.
If I pick dates like 22,06,2015 and 22,07,2015 it shows me 31 nights, which is wrong since June has only 30 days.
if I pick dates like 01,07,2015 and 31,07,2015 it shows me 30 nights, which is correct.
if I pick dates like 01,07,2015 and 1,08,2015 it shows me 31 nights etc.
if I pick dates like 30,09,2015 and 30,10,2015 it shows me 31.041666666666668 nights which is odd and incorrect.
Hope you can help me with this one. Here's the code:
var date11 = $("#in").val();
var date22 = $("#out").val();
// First we split the values to arrays date1[0] is the year, [1] the month and [2] the day
date111 = date11.split('-');
date222 = date22.split('-');
// Now we convert the array to a Date object, which has several helpful methods
date1 = new Date(date111[2], date111[1], date111[0]);
date2 = new Date(date222[2], date222[1], date222[0]);
// We use the getTime() method and get the unixtime (in milliseconds, but we want seconds, therefore we divide it through 1000)
date1_unixtime = parseInt(date1.getTime() / 1000);
date2_unixtime = parseInt(date2.getTime() / 1000);
// This is the calculated difference in seconds
var timeDifference = date2_unixtime - date1_unixtime;
// in Hours
var timeDifferenceInHours = timeDifference / 60 / 60;
// and finaly, in days :)
var timeDifferenceInDays = timeDifferenceInHours / 24;
Thanks a million!

You aren't subtracting 1 from the calendar month number:
date1 = new Date(date111[2], date111[1] - 1, date111[0]);
--------^^^^
Months are zero indexed. you should probably also round the result as if you cross a daylight saving boundary, the time value won't be an even number of days, it will be out by 1 hour (unless you cross two boundaries…)

Simple javascript date math... not really

I am trying to create a simple script that gives me the next recycling date based on a biweekly schedule starting on Wed Jul 6, 2011. So I've created this simple function...
function getNextDate(startDate) {
if (today <= startDate) {
return startDate;
}
// calculate the day since the start date.
var totalDays = Math.ceil((today.getTime()-startDate.getTime())/(one_day));
// check to see if this day falls on a recycle day
var bumpDays = totalDays%14; // mod 14 -- pickup up every 14 days...
// pickup is today
if (bumpDays == 0) {
return today;
}
// return the closest day which is in 14 days, less the # of days since the last
// pick up..
var ms = today.getTime() + ((14- bumpDays) * one_day);
return new Date(ms);
}
and can call it like...
var today=new Date();
var one_day=1000*60*60*24; // one day in milliseconds
var nextDate = getNextDate(new Date(2011,06,06));
so far so good... but when I project "today" to 10/27/2011, I get Tuesday 11/8/2011 as the next date instead of Wednesday 11/9/2011... In fact every day from now thru 10/26/2011 projects the correct pick-up... and every date from 10/27/2011 thru 2/28/2012 projects the Tuesday and not the Wednesday. And then every date from 2/29/2012 (leap year) thru 10/24/2012 (hmmm October again) projects the Wednesday correctly. What am I missing? Any help would be greatly appreciated..
V

The easiest way to do this is update the Date object using setDate. As the comments for this answer indicate this isn't officially part of the spec, but it is supported on all major browsers.
You should NEVER update a different Date object than the one you did the original getDate call on.
Sample implementation:
var incrementDate = function (date, amount) {
var tmpDate = new Date(date);
tmpDate.setDate(tmpDate.getDate() + amount)
return tmpDate;
};
If you're trying to increment a date, please use this function. It will accept both positive and negative values. It also guarantees that the used date objects isn't changed. This should prevent any error which can occur if you don't expect the update to change the value of the object.
Incorrect usage:
var startDate = new Date('2013-11-01T11:00:00');
var a = new Date();
a.setDate(startDate.getDate() + 14)
This will update the "date" value for startDate with 14 days based on the value of a. Because the value of a is not the same is the previously defined startDate it's possible to get a wrong value.

Expanding on Exellian's answer, if you want to calculate any period in the future (in my case, for the next pay date), you can do a simple loop:
var today = new Date();
var basePayDate = new Date(2012, 9, 23, 0, 0, 0, 0);
while (basePayDate < today) {
basePayDate.setDate(basePayDate.getDate()+14);
}
var nextPayDate = new Date(basePayDate.getTime());
basePayDate.setDate(nextPayDate.getDate()-14);
document.writeln("<p>Previous pay Date: " + basePayDate.toString());
document.writeln("<p>Current Date: " + today.toString());
document.writeln("<p>Next pay Date: " + nextPayDate.toString());
This won't hit odd problems, assuming the core date services work as expected. I have to admit, I didn't test it out to many years into the future...

Note: I had a similar issue; I wanted to create an array of dates on a weekly basis, ie., start date 10/23/2011 and go for 12 weeks. My code was more or less this:
var myDate = new Date(Date.parse(document.eventForm.startDate.value));
var toDate = new Date(myDate);
var week = 60 * 60 * 24 * 7 * 1000;
var milliseconds = toDate.getTime();
dateArray[0] = myDate.format('m/d/Y');
for (var count = 1; count < numberOccurrences; count++) {
milliseconds += week;
toDate.setTime(milliseconds);
dateArray[count] = toDate.format('m/d/Y');
}
Because I didn't specify the time and I live in the US, my default time was midnight, so when I crossed the daylight savings time border, I moved into the previous day. Yuck. I resolved it by setting my time of day to noon before I did my week calculation.