Convert Epoch Time to Date in Java
Problem
Epoch time counts seconds (or milliseconds) since 1 January 1970 at 00:00 UTC, which is how many systems store timestamps.
Convert the Unix timestamp 1773048600 into a UTC time, a Kolkata local time and an epoch value in milliseconds.
Java Program
import java.time.Instant;
import java.time.ZoneId;
import java.time.format.DateTimeFormatter;
import java.util.Locale;
public class ConvertEpochToDate {
public static void main(String[] args) {
long epochSeconds = 1_773_048_600L; // seconds since 1970-01-01T00:00:00Z
Instant instant = Instant.ofEpochSecond(epochSeconds);
DateTimeFormatter f = DateTimeFormatter.ofPattern("d MMM yyyy, HH:mm", Locale.ENGLISH);
System.out.println("UTC: " + instant);
System.out.println("Kolkata: " + instant.atZone(ZoneId.of("Asia/Kolkata")).format(f));
System.out.println("Millis: " + instant.toEpochMilli());
}
}Output
Core Logic
Instant.ofEpochSecond turns the number into a point on the timeline, and a zone turns that point into a local clock reading.
- 1
Instant.ofEpochSecond(epochSeconds)creates the instant. - 2Printing the
Instantshows it in UTC (theZsuffix). - 3
atZone(ZoneId.of("Asia/Kolkata"))converts it to Indian time, which is 5 hours 30 minutes ahead. - 4
toEpochMilli()gives the same moment in milliseconds.
Key Point: Check the unit: JavaScript and many APIs use milliseconds — passing them to ofEpochSecond gives a date thousands of years away.
Key Concepts
Approach 2: Converting a Date-Time to Epoch
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
public class ConvertDateToEpoch {
public static void main(String[] args) {
LocalDateTime meeting = LocalDateTime.of(2026, 3, 9, 9, 30);
long utcSeconds = meeting.toEpochSecond(ZoneOffset.UTC); // as if the time were UTC
long kolkataSeconds = meeting.atZone(ZoneId.of("Asia/Kolkata")).toEpochSecond(); // as Kolkata local time
System.out.println("Epoch seconds (read as UTC): " + utcSeconds);
System.out.println("Epoch seconds (read as Kolkata): " + kolkataSeconds);
System.out.println("Difference in hours: " + (utcSeconds - kolkataSeconds) / 3600.0);
}
}
Output
Core Logic
A LocalDateTime has no zone, so converting it to epoch time requires saying which zone the clock reading belongs to.
- 1
toEpochSecond(ZoneOffset.UTC)treats 09:30 as UTC. - 2
atZone(Kolkata).toEpochSecond()treats it as Kolkata time. - 3The two results differ by exactly 5.5 hours.
Key Point: The same wall-clock time means different instants in different zones.
Key Concepts
Interview Questions
What is the "year 2038 problem"?
Systems storing epoch seconds in a signed 32-bit integer overflow on 19 January 2038. Java's long and Instant don't have this limit.
How do you get the current epoch time in Java?
Instant.now().getEpochSecond() for seconds, or System.currentTimeMillis() for milliseconds.