Java Cookbook

Converting Epoch Seconds to DMYHMS

Problem

You need to convert a number of seconds since 1970 into a Date.

Solution

Just use the Date constructor.

Discussion

“The Epoch” is the time at the beginning of time as far as modern operating systems go. Unix time, and some versions of MS-Windows time, count off inexorably the seconds since the epoch. On systems that store this in a 32-bit integer, time is indeed running out. Let’s say we wanted to find out when the Unix operating system, whose 32-bit versions use a 32-bit date, will get into difficulty. We take a 32-bit integer of all ones, and construct a Date around it. The Date constructor needs the number of milliseconds since 1970, so we multiply by 1,000:

/** When does the UNIX date get into trouble? */

public class Y2038 {
    public static void main(String[] a) {

        // This should yield 2038AD, the hour of doom for the
        // last remaining 32-bit UNIX systems (there will be
        // millions of 64-bit UNIXes by then).

        long expiry = 0x7FFFFFFFL * 1000;

        System.out.println("32-bit UNIX expires on " +
            Long.toHexString(expiry) + " or " +
            new java.util.Date(expiry));
        // Why doesn't it?

        // Try going from msec of current time into a Date
        long now = System.currentTimeMillis(  );
        System.out.println(
            "Passing " + Long.toHexString(now) + " --> " +
            new java.util.Date(now));
        
    }
}

Sure enough, the program reports that 32-bit Unixes will expire in the year 2038 (you might think I knew that in advance if you were to judge by the name I gave the class; in fact, my ...