File lastModified() returns Wed Dec 31 19:00:00 EST 1969

Question

I was doing some testing with files like this:

    public Date findFileDate(){
    File file = new File(filePath);
    Date date = new Date(file.lastModified());
    return date;
}

When I print date it says: Wed Dec 31 19:00:00 EST 1969. After some research I found that is my "time since the Unix Epoch" according to my time zone, but I am confused why I would get this output when no file exists at my filePath. Why would it not return null or 0?

Answer 1

No, file.lastModified() is returning 0. That's the Unix epoch

In your particular time zone (Eastern US by the looks of it), local time at the Unix epoch was 5 hours behind UTC, so it was 7pm on December 31st 1969.

To confirm this, just separate your Date declaration and assignment into two:

long lastModifiedMillis = file.lastModified();
Date date = new Date(lastModifiedMillis);

Now if you examine lastModifiedMillis I'm sure you'll find a value of 0, as documented:

Returns
A long value representing the time the file was last modified, measured in milliseconds since the epoch (00:00:00 GMT, January 1, 1970), or 0L if the file does not exist or if an I/O error occurs

Answer 2

java.time

The java.util.Date object is not a real date-time object like the modern date-time types; rather, it represents the number of milliseconds since the standard base time known as "the epoch", namely January 1, 1970, 00:00:00 GMT (or UTC). When you print an object of java.util.Date, its toString method returns the date-time in the JVM's timezone, calculated from this milliseconds value. If you need to print the date-time in a different timezone, you will need to set the timezone to SimpleDateFormat and obtain the formatted string from it.

Note that the legacy date-time API (java.util date-time types and their formatting API, SimpleDateFormat) is outdated and error-prone. It is recommended to stop using it completely and switch to java.time, the modern date-time API^*.

Demo using java.time, modern date-time API:

import java.time.Instant;
import java.time.ZoneId;
import java.time.ZonedDateTime;

public class Main {
    public static void main(String args[]) {
        Instant instant = Instant.ofEpochMilli(0);
        System.out.println(instant);

        // If you need the corresponding date-time representing your timezone
        ZonedDateTime zdt = ZonedDateTime.ofInstant(instant, ZoneId.of("America/New_York"));
        System.out.println(zdt);
    }
}

Output:

1970-01-01T00:00:00Z
1969-12-31T19:00-05:00[America/New_York]

An Instant represents an instantaneous point on the time-line. The Z in the output stands for Zulu which represents UTC (timezone offset of +00:00 hours).

Learn more about the the modern date-time API^* from Trail: Date Time.

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^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}