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I am wanting to adjust 3 Gregorian Calendar dates in Java, with one to be 24 hours before, the other 48, hours before and last 78 hours before. I had been using Calendar.HOUR and changed this to Calendar.HOUR_OF_DAY.

Since I did this my code stopped working. I am comparing the adjusted dates with their original values using a method that uses date1.before(date2) and date1.after(date2) to get a comparisonflag which can be 1 or 0 which I then use in my code.

I was wondering how to do the adjust the HOUR_OF_DAY in my dates to then achieve the above.

AstroCB
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daveb
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2 Answers2

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Some code would have been nice. But if I understand the problem correctly:

From the javadoc of Calendar:

HOUR is used for the 12-hour clock. E.g., at 10:04:15.250 PM the HOUR is 10.
HOUR_OF_DAY is used for the 24-hour clock. E.g., at 10:04:15.250 PM the HOUR_OF_DAY is 22.

When adding/substracting hours from a date:

Calendar cal = Calendar.getInstance();
cal.add(Calendar.HOUR, -24);
cal.add(Calendar.HOUR_OF_DAY, -24);

This should have the same effect.

Boosty
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1

tl;dr

ZonedDateTime.now()             // Capture the current moment as seen through the lens of wall-clock time used by the people of a certain region, a time zone. Better to pass the expected/desired time zone as an optional argument.
             .minusHours( 24 )  // Do the math, get earlier moment.

Do not use Calendar

The troublesome Calendar class and related date-time classes bundled with the earliest versions of Java are now legacy, supplanted by the java.time classes.

java.time

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.

If no time zone is specified, the JVM implicitly applies its current default time zone. That default may change at any moment, so your results may vary. Better to specify your desired/expected time zone explicitly as an argument.

Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!). 

ZoneId z = ZoneId.of( "America/Montreal" ) ;

Use that time zone when asking for the current moment to be captured as a ZonedDateTime object.

ZonedDateTime zdt = ZonedDateTime.now( z ) ;

If you want to use the JVM’s current default time zone, ask for it and pass as an argument. If omitted, the JVM’s current default is applied implicitly. Better to be explicit.

ZoneId z = ZoneId.systemDefault() ;  // Get JVM’s current default time zone.

Subtract your target number of hours.

ZonedDateTime zdtMinus24h = zdt.minusHours( 24 ) ;
ZonedDateTime zdtMinus48h = zdt.minusHours( 48 ) ;
ZonedDateTime zdtMinus72h = zdt.minusHours( 72 ) ;

Compare with isBefore, isAfter, and isEqual methods. Also, Comparable is implemented.

Alternatively, you can represent the number-of-hours-to-add as a Duration.

Duration d = Duration.ofHours( 24 ) ;
ZonedDateTime zdtEarlier = zdt.minus( d ) ;

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Basil Bourque
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