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Convert JavaScript date to Swift JSON timeIntervalSinceReferenceDate format

Given a JavaScript date, how can I convert this to the same format as Swift JSON encoding?
e.g. I would like to get a value of 620102769.132999 for the date 2020-08-26 02:46:09

The default Swift JSON encoding outputs a value which is the number of seconds that have passed since ReferenceDate. https://developer.apple.com/documentation/foundation/jsonencoder/2895363-dateencodingstrategy
It seems ReferenceDate is 00:00:00 UTC on 1 January 2001.
https://developer.apple.com/documentation/foundation/nsdate/1409769-init
function dateToSwiftInterval(date: Date): number {
const referenceDate = Date.UTC(2001,0,1);
const timeSpanMs = (date - referenceDate);
return timeSpanMs / 1000;
}
const myDate = new Date(1598366769000);
console.log(dateToSwiftValue(myDate)); // 620102769

As Elwyn says, Swift represents dates as time intervals that are seconds since 1 Jan 2001 UTC. Javascript Dates use milliseconds since 1 Jan 1970 UTC, so all you need to do is adjust by the reference date difference and divide by 1000, e.g.
// Javascript function to convert a Date to a Swift time interval
// date is a Javascript Date, defaults to current date and time
function toSwiftTI(date = new Date()) {
return (date - Date.UTC(2001,0,1)) / 1000;
}
console.log(toSwiftTI());
Since the time difference is a constant, 978307200000, you might just use that instead of calculating it every time, so:
return (date - 978307200000) / 1000;
Going the other way, just multiply by 1,000 and add the constant:
function swiftToDate(ti) {
return new Date(ti * 1000 + 978307200000);
}
console.log(swiftToDate(620102769.132999).toISOString());

How do I compare a Date string in EST with a Date object in UTC?

I'm trying to compare this string 8/26/2019 6:53:13 which is in EST to a new Date() object to simply see if it's in the past. This works fine locally but on deployment heroku's new Date comes through in UTC format. So I had to do this hack
if(process.env.NODE_ENV){
timeSubmitted.setHours(timeSubmitted.getHours() + 5); // HACK but does work
}
I tried to get todays date and time in UTC format as an object not a string so I can compare it to other times in UTC format.
var date = new Date('2014-02-27T10:00:00')
//Thu Feb 27 2014 10:00:00 GMT-0500 (Eastern Standard Time) //this is an object
let d = moment.utc(new Date()).format()
//Does convert right now to a UTC time string, but then when I try convert it to an object
new Date(d)
//it goes back to EST
All together this does work, but isn't ideal because of the hardcoded number 5.
//SET DATE RANGE
const startDate = new Date();//This gets converted from EST to UTC
startDate.setMinutes(startDate.getMinutes() - 2); //Google spreadsheets saves minutes a little in the past
//startDate = new Date(startDate.getTime() + startDate.getTimezoneOffset() * 60000);
const endDate = new Date();
endDate.setDate(endDate.getDate() + 5)
console.log('startDate ' ,startDate,'endDate ',endDate)
rows.forEach(row=>{
//row.timestamp looks like this '8/26/2019 6:53:13' in EST
var date= row.timestamp.split(" ")[0].split('/')
var time=row.timestamp.split(" ")[1].split(':')
var timeSubmitted = new Date(date[2], date[0] - 1, date[1], time[0], time[1], time[2]); //So this is in EST
//but then when deploying to heroku i had to do this hack.
if(process.env.NODE_ENV){
timeSubmitted.setHours(timeSubmitted.getHours() + 5); // HACK -- It's the only way I could get this time to be in UTC/timeSubmitted = new Date(timeSubmitted.getTime() + timeSubmitted.getTimezoneOffset() * 60000);
}
console.log('timeSubmitted',typeof timeSubmitted, typeof startDate, timeSubmitted, startDate, timeSubmitted >= startDate, timeSubmitted < endDate)
if(timeSubmitted >= startDate && timeSubmitted < endDate){ //Within the date range so check if the names are in the roster
slackers = slackers.filter(function(a){return a.fullname !== row.whatsyourname})
}
})
messageSlackers(slackers, id)

Timezones are just a factor taken into consideration when generating a human-readable string from a date.
But a date is a point in time, regardless of timezones. Time doesn't know about timezones! Humans invented timezones.
You're probably stringising your Date objects in some manner that uses your local timezone by default (didn't show us this code). This doesn't matter.
Comparing two Dates works. Always. You don't have to worry about some hidden timezone component ruining it. Just compare your dates and you'll see that it works fine.
tl;dr: Dates are not in a "format". Dates are dates.

setHours is not a function when converting time from string

I am trying to compare a time in string format to the current time. I've tried setting up two Date objects and calling .Now() on both of them, then on one of them adjusting the time to the time that is in string format by splitting it and parsing both the hours and minutes to integers, but I get the following error:
setHours is not a function
The 'cutoff' value I'm using is '15:00' and when following in the debugger I can see this splits in to split[0] = 15 and split[1] = 00 (this is before they are parsed into integers.
var cutoff = data.CutOff;
var split = cutoff.split(":");
var today = Date.now();
var hours = parseInt(split[0]);
var min = parseInt(split[1]);
today.setHours(hours, min);
if (Date.now() < today) {
// Do Something
}

You want to do new Date() as opposed to Date.now()
new Date creates a Date instance which allows you to access the Date methods.
Date.now() method returns the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC.

How to the get the beginning of day of a date in javascript -- factoring in timezone

I am struggling to find out the beginning of day factoring in timezones in javascript. Consider the following:
var raw_time = new Date(this.created_at);
var offset_time = new Date(raw_hour.getTime() + time_zone_offset_in_ms);
// This resets timezone to server timezone
var offset_day = new Date(offset_time.setHours(0,0,0,0))
// always returns 2011-12-08 05:00:00 UTC, no matter what the offset was!
// This has the same issue:
var another_approach_offset_day = new Date(offset_time.getFullYear(),offset_time.getMonth(),offset_time.getHours())
I expect when i pass a Pacific Timezone offset, to get: 2011-12-08 08:00:00 UTC and so on.
What is the correct way to achieve this?
I think that part of the issue is that setHours method sets the hour (from 0 to 23), according to local time.
Also note that I am using javascript embedded in mongo, so I am unable to use any additional libraries.
Thanks!
Jeez, so this was really hard for me, but here is the final solution that I came up with the following solution. The trick was I need to use setHours or SetUTCHours to get the beginning of a day -- the only choices I have are system time and UTC. So I get the beginning of a UTC day, then add back the offset!
// Goal is given a time and a timezone, find the beginning of day
function(timestamp,selected_timezone_offset) {
var raw_time = new Date(timestamp)
var offset_time = new Date(raw_time.getTime() + selected_timezone_offset);
offset_time.setUTCHours(0,0,0,0);
var beginning_of_day = new Date(offset_time.getTime() - selected_timezone_offset);
return beginning_of_day;
}

In JavaScript all dates are stored as UTC. That is, the serial number returned by date.valueOf() is the number of milliseconds since 1970-01-01 00:00:00 UTC. But, when you examine a date via .toString() or .getHours(), etc., you get the value in local time. That is, the local time of the system running the script. You can get the value in UTC with methods like .toUTCString() or .getUTCHours(), etc.
So, you can't get a date in an arbitrary timezone, it's all UTC (or local). But, of course, you can get a string representation of a date in whatever timezone you like if you know the UTC offset. The easiest way would be to subtract the UTC offset from the date and call .getUTCHours() or .toUTCString() or whatever you need:
var d = new Date();
d.setMinutes(d.getMinutes() - 480); // get pacific standard time
d.toUTCString(); // returns "Fri, 9 Dec 2011 12:56:53 UTC"
Of course, you'll need to ignore that "UTC" at the end if you use .toUTCString(). You could just go:
d.toUTCString().replace(/UTC$/, "PST");
Edit: Don't worry about when timezones overlap date boundaries. If you pass setHours() a negative number, it will subtract those hours from midnight yesterday. Eg:
var d = new Date(2011, 11, 10, 15); // d represents Dec 10, 2011 at 3pm local time
d.setHours(-1); // d represents Dec 9, 2011 at 11pm local time
d.setHours(-24); // d represents Dec 8, 2011 at 12am local time
d.setHours(52); // d represents Dec 10, 2011 at 4am local time

Where does the time_zone_offset_in_ms variable you use come from? Perhaps it is unreliable, and you should be using Date's getTimezoneOffset() method. There is an example at the following URL:
http://www.w3schools.com/jsref/jsref_getTimezoneOffset.asp

If you know the date from a different date string you can do the following:
var currentDate = new Date(this.$picker.data('date'));
var today = new Date();
today.setHours(0, -currentDate.getTimezoneOffset(), 0, 0);
(based on the codebase for a project I did)

var aDate = new Date();
var startOfTheDay = new Date(aDate.getTime() - aDate.getTime() % 86400000)
Will create the beginning of the day, of the day in question

You can make use of Intl.DateTimeFormat. This is also how luxon handles timezones.
The code below can convert any date with any timezone to its beginging/end of the time.
const beginingOfDay = (options = {}) => {
const { date = new Date(), timeZone } = options;
const parts = Intl.DateTimeFormat("en-US", {
timeZone,
hourCycle: "h23",
hour: "numeric",
minute: "numeric",
second: "numeric",
}).formatToParts(date);
const hour = parseInt(parts.find((i) => i.type === "hour").value);
const minute = parseInt(parts.find((i) => i.type === "minute").value);
const second = parseInt(parts.find((i) => i.type === "second").value);
return new Date(
1000 *
Math.floor(
(date - hour * 3600000 - minute * 60000 - second * 1000) / 1000
)
);
};
const endOfDay = (...args) =>
new Date(beginingOfDay(...args).getTime() + 86399999);
const beginingOfYear = () => {};
console.log(beginingOfDay({ timeZone: "GMT" }));
console.log(endOfDay({ timeZone: "GMT" }));
console.log(beginingOfDay({ timeZone: "Asia/Tokyo" }));
console.log(endOfDay({ timeZone: "Asia/Tokyo" }));

Python Unix time doesn't work in Javascript

In Python, using calendar.timegm(), I get a 10 digit result for a unix timestamp. When I put this into Javscript's setTime() function, it comes up with a date in 1970. It evidently needs a unix timestamp that is 13 digits long. How can this happen? Are they both counting from the same date?
How can I use the same unix timestamp between these two languages?
In Python:
In [60]: parseddate.utctimetuple()
Out[60]: (2009, 7, 17, 1, 21, 0, 4, 198, 0)
In [61]: calendar.timegm(parseddate.utctimetuple())
Out[61]: 1247793660
In Firebug:
>>> var d = new Date(); d.setTime(1247793660); d.toUTCString()
"Thu, 15 Jan 1970 10:36:55 GMT"

timegm is based on Unix's gmtime() method, which return seconds since Jan 1, 1970.
Javascripts setTime() method is milliseconds since that date. You'll need to multiply your seconds times 1000 to convert to the format expected by Javascript.

Here are a couple of python methods I use to convert to and from javascript/datetime.
def to_datetime(js_timestamp):
return datetime.datetime.fromtimestamp(js_timestamp/1000)
def js_timestamp_from_datetime(dt):
return 1000 * time.mktime(dt.timetuple())
In javascript you would do:
var dt = new Date();
dt.setTime(js_timestamp);

Are you possibly mixing up seconds-since-1970 with milliseconds-since-1970?

JavaScript Date constructor works with milliseconds, you should multiply the Python unix time by 1000.
var unixTimestampSeg = 1247793660;
var date = new Date(unixTimestampSeg*1000);