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How can i convert date to timestamp or preferably how can i add n number of days to a timestamp in javascript

I am working on firebase cloud functions that i'm new to , using javascript. I have a need to add n number of days to a user who has renewed a subscription.
Here is part of the function
...
var sellerRef = admin.firestore().doc('sellerProfile/'+req.query.sellerId)
var seller = sellerRef.get().then(snapshot=>{
const data= snapshot.data()
var currentDate = data.subscriptionDeadline.toDate()
Date.prototype.addDays= function(d){
this.setDate(this.getDate() + d);
return this;
};
var newDate = currentDate.addDays(30);
res.send(newDate)
})
sellerRef.update({
subscriptionDeadline: Date.parse(newDate)
})
...
In that code i got the subscription deadline of an individual then used a prototype to add 30 days as a subscription renewal. How do i convert the days back to timestamp to be able to save the new subscription deadline to firestore or better if i could add the 30 days without converting the timestamp to date.

Firestore Timestamps objects don't offer any date math operations. It will be easier if you do convert the Timestamp into a Date (or something else that lets you do math), the convert back to a Timestamp.
const timestamp = data.subscriptionDeadline;
const date = timestamp.toDate();
const laterDate = new Date(date.getTime() + 30*60*60*1000);
const laterTimestamp = admin.firestore.Timestamp.fromDate(laterDate);

SQL date discrepancy with some dates

I am working on a date problem.
So, in the app I have a table that have some user records that includes a date of birth field (datetime sql type). The problem is that for some users whose date of birth is prior to 1954, the date is not properly reflected.
For example, I have a user whose date of birth is 11/08/1920 but when I set the date of birth via server script, it ends up with the value 11/07/1920 23:23:24.
I am getting the date value from a spreadsheet and the server script looks like this:
function importDOBs(){
var allRecs = [];
var ss = SpreadsheetApp.openById("adfsasdfasdfasdfasdfasdf");
var sheet = ss.getActiveSheet();
var data = sheet.getRange(2,4,sheet.getLastRow(),2).getValues();
for(var i=0; i<5; i++){
var row = data[i];
var date = row[0];
var oldMrn = row[1];
var query = app.models.purgedRecords.newQuery();
query.filters.oldMrn._equals = oldMrn;
var record = query.run()[0];
if(record){
var dob = new Date(date);
record.dateOfBirth = dob;
allRecs.push(record);
}
}
app.saveRecords(allRecs);
}
These are the values in the spreadsheet (they are strings, not dates):
1954-03-04T00:00:00
2014-03-01T00:00:00
1951-10-20T00:00:00
1920-11-08T00:00:00
1938-09-27T00:00:00
However, somehow I'm always getting this:
As you see, there is a discrepancy on the dates that are prior to 1954. So far, I have tried other things such as changing this part:
if(record){
var dob = new Date(date);
record.dateOfBirth = dob;
allRecs.push(record);
}
to this:
if(record){
var dob = Utilities.formatDate(new Date(date),"GMT","yyyy-MM-dd'T'HH:mm:ss'Z'");
var birthDate = new Date(dob);
record.dateOfBirth = birthDate;
allRecs.push(record);
}
and the above resulted in the same thing. I have tried other dates after 1954 and they also seem wrong. For example 05/19/1968 reflects 05/18/1968 23:00:00. So my best guess so far this has to do something with the daylight savings, perhaps?

The snake pit of converting dates between platforms
Try to set the values to UTC or just format them into the datetime format for MySQL
The first option as seen in the snippet below requires you to convert the format in SQL:
See this answer: MySQL yyyy-mm-ddThh:mm:ss.sssZ to yyyy-mm-dd hh:mm:ss
DATE_FORMAT(STR_TO_DATE(«string»,'%Y-%m-%dT%H:%i:%s.000Z'),'%Y-%m-%d %H:%i:%s');
//Using Z to set to UTC
let allRecs = [];
document.querySelector("pre").textContent.split("\n").forEach(function(element) {
if (element != "") {
const dob = new Date(element + "Z"); //Z forces te date to be in UTC with zero time offzet or: 00:00:00
const dateOfBirth = dob;
allRecs.push(dateOfBirth);
}
});
console.log(allRecs);
allRecs = [];
//format to MySQL datetime format
document.querySelector("pre").textContent.split("\n").forEach(function(element) {
if (element != "") {
element = element.replace("T", " "); //replace T with space
//now the string is in the datetime format for MySQL
allRecs.push(element);
}
});
console.log(allRecs);
<pre>
1954-03-04T00:00:00
2014-03-01T00:00:00
1951-10-20T00:00:00
1920-11-08T00:00:00
1938-09-27T00:00:00
</pre>

Date in variable reading as "Invalid Date"

Using the following:
var timestart = $('.thisDiv').data("timestart");
var startDateTime = new Date(timestart);
to collect a date from a php file that is updating by ajax from this:
$TimeStart = date( 'Y,m,d,g,i', $TimeStart );
<div class="thisDiv" data-timestart="<?= $TimeStart ?>"></div>
var timestart = $('.thisDiv').data("timestart");
In console I'm getting the following when logging timestart and startDateTime:
2017,07,24,7,50
Invalid Date
If I paste the date that is output as follows
var startDateTime = new Date(2017,07,24,7,50);
Then it works fine. Any ideas why I'm getting Invalid Date?

Your timestart variable (JavaScript) is just a string. So it's a string 2017,07,24,7,50, and not those elements in order - which can't be used as separate parameters like new Date() expects.
Let's take a look at it!
var startDateTime = new Date(2017,07,24,7,50); // Parameters in order - all OK!
var startDateTime = new Date("2017,07,24,7,50"); // A single string - single parameter, not OK!
You need to return a proper format of dates from PHP with a format that's valid in JavaScript. Per the ECMAScript standard, the valid format that should work across all browsers is YYYY-MM-DDTHH:mm:ss.sssZ (see the reference at the bottom). To define that from PHP, you would need to format it as such
$TimeStart = date('c', $TimeStart);
This would return a format such as 2017-07-24T21:08:32+02:00.
Alternatively, you can use a splat/spread-operator ... and split the string it into elements, which I find as the better approach than above.
var timestart = $('.thisDiv').data("timestart"); // Get the string: "2017,07,24,7,50"
timestart = timestart.split(","); // Split into array
var startDateTime = new Date(...timestart); // Pass as arguments with splat-operator
Spread/splat operator in JavaScript
PHP date() documentation
ECMAScript date string (JavaScript)

You need to convert your date from string format to numbers.
var timestart = $('.thisDiv').data("timestart").split(",").map(Number);
var startDateTime = new Date(...timestart);
console.log(startDateTime)
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="thisDiv" data-timestart="2017,07,24,7,50"></div>

Google Form on Submit get values and format the time

I am using Google Apps Script with a Google form. When the user submits the Google Form I get a value from a question. I then take that value and make it a date object, from what I saw on this post about daylight savings I use that to determine the timezone. I run the date object through Utilities.formatDate and want to get the correctly formatted date.
example: 9:00 AM
But instead I am getting a completely different time than expected.
My question is: Can someone help me understand why the below code is outputting a time that is 3 hours different?
function onSubmit(e) {
var values = e.values;
Logger.log(values);
try {
var start1 = new Date(values[3]);
var startN = new Date(start1).toString().substr(25,6)+"00";
var startT = Utilities.formatDate(start1, startN, "h:mm a");
Logger.log(startT);
} catch(error) {
Logger.log(error);
}
}

The assumption that Utilities formatDate does not support GMT... parameter is not true.
The post you mentioned in reference is used to get calendar events and is a useful way to get the right value when you get events from another daylight saving period (getting the TZ info from the calendar event itself), for example events for next month will be in "summer time" while we are still in "winter time"...
Your issue might come from different sources depending on time zone settings of your script vs timezone of the source. Could you describe the exact configuration in which you use this script ?
In the mean time, here is a small code that demonstrates how the code is working + the logger results :
function testOnSubmit() {
var eventInfo = {};
var values = {};
values['3'] = new Date();
eventInfo['values'] = values;
Logger.log('eventInfo = '+JSON.stringify(eventInfo)+'\n\n');
onSubmit(eventInfo);
}
function onSubmit(e) {
var values = e.values;
try {
var start1 = new Date(values[3]);
Logger.log('onSubmit log results : \n');
Logger.log('start1 = '+start1)
var startN = new Date(start1).toString().substr(25,6)+"00";
Logger.log('startN = '+startN);
var startT = Utilities.formatDate(start1, startN, "h:mm a");
Logger.log('result in timeZone = '+startT);
} catch(error) {
Logger.log(error);
}
}
EDIT : additionally, about the 30 and 45' offset, this can easily be solved by changing the substring length like this :
var startN = new Date(start1).toString().substr(25,8);
the result is the same, I had to use the other version a couple of years ago because Google changed the Utilities.formatDate method at some moment (issue 2204) but this has been fixed.
EDIT 2 : on the same subject, both methods actually return the same result, the GMT string has only the advantage that you don't have to know the exact timezone name... there is also the Session.getScriptTimeZone() method. Below is a demo script that shows the resulst for 2 dates in January and July along with the log results :
function testOnSubmit() {
var eventInfo = {};
var values = {};
values['3'] = new Date(2014,0,1,8,0,0,0);
eventInfo['values'] = values;
Logger.log('eventInfo = '+JSON.stringify(eventInfo)+'\n\n');
onSubmit(eventInfo);
values['3'] = new Date(2014,6,1,8,0,0,0);
eventInfo['values'] = values;
Logger.log('eventInfo = '+JSON.stringify(eventInfo)+'\n');
onSubmit(eventInfo);
}
function onSubmit(e) {
var values = e.values;
var start1 = new Date(values[3]);
Logger.log('onSubmit log results : ');
Logger.log('start1 = '+start1)
var startN = new Date(start1).toString().substr(25,8);
Logger.log('startN = '+startN);
Logger.log('result in timeZone using GMT string = '+Utilities.formatDate(start1, startN, "MMM,d h:mm a"));
Logger.log('result in timeZone using Joda.org string = '+Utilities.formatDate(start1, 'Europe/Brussels', "MMM,d h:mm a"));
Logger.log('result in timeZone using Session.getScriptTimeZone() = '+Utilities.formatDate(start1, Session.getScriptTimeZone(), "MMM,d h:mm a")+'\n');
}
Note also that the Logger has its own way to show the date object value ! it uses ISO 8601 time format which is UTC value.

Try this instead:
var timeZone = Session.getScriptTimeZone();
var startT = Utilities.formatDate(start1, timeZone, "h:mm a");
The Utilities.formatDate function expects a time zone that is a valid IANA time zone (such as America/Los_Angeles), not a GMT offset like GMT+0700.
I am making the assumption that Session.getScriptTimeZone() returns the appropriate zone. If not, then you might need to hard-code a specific zone, or use a different function to determine it.
Additionally, the +"00" in the script you had was assuming that all time zones use whole-hour offsets. In reality, there are several that have 30-minute or 45-minute offsets.

How to get time difference between two date time in javascript?

I want time duration between two date time. I have the start date, start time, end date and end time. Now I have to find the difference between them.
Actually I have tried with this following code, but I got the alert like 'invalidate date'.
function myfunction()
{
var start_dt = '2013-10-29 10:10:00';
var end_dt = '2013-10-30 10:10:00';
var new_st_dt=new Date(start_dt);
var new_end_dt=new Date(end_dt);
alert('new_st_dt:'+new_st_dt);
alert('new_end_dt:'+new_end_dt);
var duration=new_end_dt - new_st_dt;
alert('duration:'+duration);
}
the alert msg like as follows:
new_st_dt:invalid date
new_end_dt: invalid date
duration:NaN
when I run in android simulator I got these alert messages.
Please help me how to get it? How to implement this?

You're passing an invalid ISO date string to that Date() constructor. It needs a form like
YYYY-MM-DDThh:mm:ss
for instance
2013-10-29T10:10:00
So you basically forgot the T to separate date and time. But even if the browser reads in the ISO string now, you would not have an unix timestamp to calculate with. You either can call
Date.parse( '2013-10-29T10:10:00' ); // returns a timestamp
or you need to explicitly parse the Date object, like
var duration=(+new_end_dt) - (+new_st_dt);
Further read: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/parse

Try formatting you timestamps as isoformat so javascript recognizes them. (you put a "T" between the date and time). An example: '2013-10-29T10:10:00'

function dateDiff(){
var start_dt = '2013-10-29 10:10:00';
var end_dt = '2013-10-30 10:10:00';
var d1= start_dt ;
d1.split("-");
var d2= end_dt ;
d2.split("-");
var t1 = new Date(d2[0],d2[1],d2[2]);
var t2 = new Date(d1[0],d1[1],d1[2]);
var dif = t1.getTime() - t2.getTime();
var Seconds_from_T1_to_T2 = dif / 1000;
return Math.abs(Seconds_from_T1_to_T2);
}