Find the number of days ago a given java.util.Date object is from the current date

Question

What's the most standard, performant way of figuring out how many days ago a particular java.util.Date object represents? Ideally, I want to get back a double representing the (potentially) fractional number of days ago.

Answer 1

That sounds remarkably like:

(System.currentTimeMillis() - date.getTime()) / (24 * 60 * 60 * 1000d);

In other words, find out the difference between the current time and the given date in millis, and then divide by the number of milliseconds in a day. I've explicitly made the 1000d a double literal to make the final division work in double arithmetic.

Answer 2

java.time

The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern Date-Time API^*.

Solution using java.time, the modern Date-Time API:

You can use java.time.Duration, which is modelled on ISO-8601 standards and was introduced with Java-8 as part of JSR-310 implementation, to find the duration between two instants. Also, avoid performing calculations yourself if you already have a standard API to do the job.

import java.time.Duration;
import java.time.Instant;

public class Main {
    public static void main(String[] args) {
        // A sample date-time in the past
        String strDateTime = "2020-09-10T10:20:30.123456789Z";

        Instant past = Instant.parse(strDateTime);
        Instant now = Instant.now();

        Duration duration = Duration.between(past, now);
        long durationInMillis = duration.toMillis();

        double days = (double) durationInMillis / Duration.ofDays(1).toMillis();
        System.out.println(days);
    }
}

Output from a sample run:

299.14329884259257

ONLINE DEMO

Learn more about 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.}

Answer 3

Get System.currentTimeMillis() and find Date from it. Then get day, month, year. Now it should be easy find difference.