Why Are Timestamps Different in Java and JavaScript?

What are the differences between timestamps in Java and JavaScript?

// Java timestamp example
long timestampInJava = System.currentTimeMillis();

// JavaScript timestamp example
let timestampInJavaScript = Date.now();

Timestamps in Java and JavaScript are generated and interpreted differently, which can lead to inconsistencies that developers need to address. Understanding these differences is crucial for applications that involve both languages, especially in full-stack development.

// Java example ensuring UTC output
import java.time.Instant;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;

ZonedDateTime nowUtc = ZonedDateTime.now(ZoneOffset.UTC);
System.out.println(nowUtc.toInstant().toEpochMilli());

// JavaScript to convert to UTC
let utcNow = new Date(Date.now()).toISOString();

Causes

• Java utilizes the Unix epoch time (in milliseconds) provided by System.currentTimeMillis() for timestamps.
• JavaScript also uses the Unix epoch; however, it frequently returns timestamps in milliseconds since January 1, 1970, UTC based on the Date.now() method.
• Timezone differences can affect the interpretation of the timestamp when converted to a human-readable format. Java strictly considers the default timezone of the JVM, while JavaScript uses the browser's timezone.

Solutions

• Ensure that both Java and JavaScript are consistently using the same unit for timestamps (either milliseconds or seconds). For example, divide by 1000 in Java if using seconds.
• When sending timestamps from Java to JavaScript, format them in UTC and use libraries like Moment.js or Day.js in JavaScript to handle conversions correctly.
• Leverage Java's ZonedDateTime class to manage timezone discrepancies effectively.

Mistake: Assuming both timestamps are in the same format or timezone.

Solution: Always explicitly convert timestamps to a standardized format such as UTC before using them.

Mistake: Using Date() constructors in JavaScript without considering local time settings.

Solution: Utilize UTC methods like Date.UTC() to avoid local timezone interference.