datetime.js

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// converting b/w JavaScript Date and Java LocalDateTime

// JavaScript Date with current time
var d = new Date();
print(d);
 
// Java 8 java.time classes used
var Instant = java.time.Instant;
var LocalDateTime = java.time.LocalDateTime;
var ZoneId = java.time.ZoneId;
 
// Date.prototype.getTime

// getTime() method returns the numeric value corresponding to the time
// for the specified date according to universal time. The value returned
// by the getTime() method is the number of milliseconds since 1 January 1970 00:00:00 UTC.
// See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getTime
 
// Java Instant.ofEpochMilli to convert time in milliseconds to Instant object
// https://docs.oracle.com/javase/8/docs/api/java/time/Instant.html#ofEpochMilli-long-
 
var instant = Instant.ofEpochMilli(d.getTime());
 
// Instant to LocalDateTime using LocalDateTime.ofInstant
// https://docs.oracle.com/javase/8/docs/api/java/time/LocalDateTime.html#ofInstant-java.time.Instant-java.time.ZoneId-
 
var ldt = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());
print(ldt);
 
// converting a LocalDateTime to JavaScript Date
// convert LocalDateTime to Instant first
// https://docs.oracle.com/javase/8/docs/api/java/time/LocalDateTime.html#atZone-java.time.ZoneId-
 
var instant = ldt.atZone(ZoneId.systemDefault()).toInstant();
 
// instant to to epoch milliseconds
// https://docs.oracle.com/javase/8/docs/api/java/time/Instant.html#toEpochMilli--
// and then to JavaScript Date from time in milliseconds
// https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Date

var d1 = new Date(instant.toEpochMilli());
print(d1);