Check if 24 hours have passed (reading from a string)

Question

I am saving date's in a file in the following format as a string.

Sat Jul 21 23:31:55 EDT 2012

How can I check if 24 hours have passed? I am a beginner so please explain it a little bit =)

Answer 1

I am not sure if I completely understood the question - do you have two dates for comparison or do you wish to keep checking periodically if 24 hours have elapsed? If comparing two date/times, I would suggest looking at joda or perhaps date4j. Using joda, one could look into using interval between two dates:

Interval interval = new Interval(previousTime, new Instant());

where previous time would be the time you mentioned

Answer 2

I would suggest storing your information as a java.util.Calendar which has a compareTo ()function.

If you want to compare now to current time, you can use System.getCurrentTimeMillis() to get the current time.

Answer 3

You can do something like this:

try {

    // reading text...
    Scanner scan = new Scanner( new FileInputStream( new File( "path to your file here..." ) ) );
    String dateString = scan.nextLine();

    // creating a formatter.
    // to understand the format, take a look here: http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html
    // EEE: Day name of week with 3 chars
    // MMM: Month name of the year with 3 chars
    // dd: day of month with 2 chars
    // HH: hour of the day (0 to 23) with 2 chars
    // mm: minute of the hour with 2 chars
    // ss: second of the minute with 2 chars
    // zzz: Timezone with 3 chars
    // yyyy: year with 4 chars
    DateFormat df = new SimpleDateFormat( "EEE MMM dd HH:mm:ss zzz yyyy", Locale.US );

    // parsing the date (using the format above, that matches with your date string)
    Date date = df.parse( dateString );

    // now!
    Date now = new Date();

    // gets the differente between the parsed date and the now date in milliseconds
    long diffInMilliseconds = now.getTime() - date.getTime();

    if ( diffInMilliseconds < 0 ) {
        System.out.println( "the date that was read is in the future!" );
    } else {

        // calculating the difference in hours
        // one hour have: 60 minutes or 3600 seconds or 3600000 milliseconds
        double diffInHours = diffInMilliseconds / 3600000D;
        System.out.printf( "%.2f hours have passed!", diffInHours );

    }

} catch ( FileNotFoundException | ParseException exc ) {
    exc.printStackTrace();
}

Answer 4

Define A Day

Do you really mean one day or 24-hours? Because of Daylight Saving Time nonsense, a day can vary in length such as 23 or 25 hours in the United States.

Avoid 3-Letter Time Zone Codes

That String format is a terrible representation of a date-time. It is difficult to parse. It uses a 3-letter time zone code, and such codes are neither standardized nor unique. If possible, choose another format. The obvious choice is ISO 8601, for example: 2014-07-08T04:17:01Z.

Use proper time zone names.

Avoid j.u.Date & .Calendar

The java.util.Date and .Calendar classes bundled with Java are notoriously troublesome. Avoid them.

Instead use either the venerable Joda-Time library or the new java.time package bundled in Java 8 (and inspired on Joda-Time).

Joda-Time

Here is some example code in Joda-Time.

Get the current moment.

DateTime now = DateTime.now();

Parse the input string.

String input = "Sat Jul 21 23:31:55 EDT 2012";
DateTime formatter = DateTimeFormat.forPattern( "EEE MMM dd HH:mm:ss zzz yyyy" ).with Locale( java.util.Locale.ENGLISH );
DateTime target = formatter.parseDateTime( input );

Calculate 24 hours (or next day).

DateTime twentyFourHoursLater = target.plusHours( 24 );

Test if current moment happened after.

boolean expired = now.isAfter( twentyFourHoursLater );

Or, if you want next day rather than 24-hours, use plusDays rather than plusHours. If necessary, adjust to desired time zone. Time zone is crucial as it defines the day/date and applies rules for anomalies such as Daylight Saving Time.

DateTime targetAdjusted = target.withZone( DateTimeZone.forID( "Europe/Paris" ) );
…
DateTime aDayLater = targetAdjusted.plusDays( 1 ); // Go to next day, accounting for DST etc.
boolean expired = now.isAfter( aDayLater ); // Test if current moment happened after.