Javascript - how to convert decimal value to datetime

Question

I am trying to convert a decimal number to a date object in Javascript. I have tried multiple methods and using moment, but all my conversions are wrong. An example of a value I want to convert is 705623516.402048. I have tried to interpret the int part as seconds and the decimal part as milliseconds, but it has gone wrong too.

I cannot use JQuery, just plain JS or moment

Any help is greatly appreciated

Thanks in advance

You said that the int part is seconds. Seconds from when? Normally the base is 1970-01-01. But when I use that, I get a date in 1992. — Barmar, May 12 '23 at 22:47
I don't know, I just receive the number from an outside source, it is supposed to represent current date time — Rodolfo Conde, May 12 '23 at 22:48
You need to know. There's no way to interpret it without that information. — Barmar, May 12 '23 at 22:49
I did some calculations and it appears to be seconds since 2001-01-01 00:00:00. — Barmar, May 12 '23 at 22:51

score 2 · Accepted Answer · answered May 12 '23 at 22:54

2

If that's supposed to be today's date, it looks like it's seconds since 2001-01-01. So you can use Date.setSeconds() to add to that date.

let seconds = 705623516.402048;
let date = new Date('2001-01-01 00:00:00');
date.setSeconds(date.getSeconds() + seconds);
console.log(date);

answered May 12 '23 at 22:54

Barmar

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I am doing right now the test and it works !!!! Thanks a lot – Rodolfo Conde May 12 '23 at 23:18
Epochs are typically UTC, so `let date = new Date(Date.UTC(2001,0))` might be a better choice. ;-) – RobG May 15 '23 at 13:20

score 0 · Answer 2 · answered May 15 '23 at 13:25

The time value appears to be microseconds. Based on Barmar's answer, you can maintain millisecond precision (the best ECMAScript Dates can do currently) using:

let seconds = 705623516.402048;
let date = new Date(Date.UTC(2001, 0) + seconds * 1e3);

console.log(date);

Date epochs are typically UTC (though not certainly), hence the use of Date.UTC.

Javascript - how to convert decimal value to datetime

2 Answers2