java.time.Instant response to JavaScript date

Question

I have a Spring Boot server that returns me a date as follows:

{
  // Some keys
  pickupDate: {
    epochSecond: 1612199331,
    nano: 428000000
  },
  // Some other keys
}

How can I convert that object to a JavaScript Date?

Answer 1

tl;dr

new Date( ( 1_612_199_331 * 1_000 ) + ( 428_000_000 / 1_000_000 ) ) 

Java

If those values come from objects in the java.time classes of Java 8 and later such as Instant , then:

• epochSecond represents a count of whole seconds since the epoch reference of the first moment of 1970 as seen in UTC, 1970-01-01T00:00Z.
• nano represents a fractional second as a count of nanoseconds , billionths of a second.

JavaScript

While I do not know JavaScript, it seems that most implementations offer a Date type. A Date represents a moment as seen in UTC based on a count from the same epoch reference as java.time .

The difference is the granularity of the fractional second. The JavaScript Date uses milliseconds rather the nanoseconds in java.time . So you will need to divide that count of nanos by 1,000,000 to get millis.

Multiply the whole seconds by a thousand to get milliseconds. And divide the count of nanos by a million to get milliseconds. Sum to get a total number of milliseconds since epoch reference. Pass to constructor of Date .

const d = 
        new Date( 
          ( 1_612_199_331 * 1_000 ) 
          + 
          ( 428_000_000 / 1_000_000 ) 
        )
;

Generate text representing the moment stored in that Date using standard ISO 8601 format.

const d = new Date( ( 1_612_199_331 * 1_000 ) + ( 428_000_000 / 1_000_000 ) ) ;
console.log( d.toISOString() ) ;

2021-02-01T17:08:51.428Z

The Z on the end means an offset of zero hours-minutes-seconds from UTC. Pronounced “Zulu”.