How to Retrieve Milliseconds from Epoch (January 1, 1970) in Java

How can I obtain the current milliseconds from the epoch (January 1, 1970) in Java and how can I calculate the milliseconds for any given date/time in UTC?

In Java, you can retrieve the current time in milliseconds since the epoch (January 1, 1970) using the `System.currentTimeMillis()` method. Additionally, to get the number of milliseconds for any specific date and time in UTC, you can utilize the `java.time` package introduced in Java 8, which provides a simplified approach to date and time manipulation.

import java.time.Instant;\nimport java.time.ZonedDateTime;\n\npublic class EpochTimeExample {\n    public static void main(String[] args) {\n        // Get current milliseconds from epoch\n        long currentEpochMillis = System.currentTimeMillis();\n        System.out.println("Current milliseconds since epoch: " + currentEpochMillis);\n\n        // Get milliseconds from epoch for a specific date (e.g., 2023-10-15T12:00:00Z)\n        ZonedDateTime specificDateTime = ZonedDateTime.parse("2023-10-15T12:00:00Z");\n        long specificMillis = specificDateTime.toInstant().toEpochMilli();\n        System.out.println("Milliseconds from epoch for 2023-10-15T12:00:00Z: " + specificMillis);\n    }\n}

Causes

• The epoch time is the standard reference point used for computing time in computing since January 1, 1970.
• Milliseconds representation allows for precise time tracking and operations.

Solutions

• Use `System.currentTimeMillis()` to get the current time in milliseconds from epoch.
• For a specific date/time, use `ZonedDateTime` and its `toInstant()` method to convert it to milliseconds from epoch.

Mistake: Not accounting for time zone differences when converting local date/time to epoch milliseconds.

Solution: Always use UTC date-time formats when dealing with epoch time.

Mistake: Using outdated Date and Calendar classes which are less efficient and more error-prone.

Solution: Utilize the java.time package, specifically ZonedDateTime and Instant, for better date/time handling.