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How to do a date add or substract?

The java.util.Calendar allows us to do a date arithmetic function such as add or substract a unit of time to the specified date field.

The method that done this process is the Calendar.add(int field, int amount). Where the value of the field can be Calendar.DATE, Calendar.MONTH, Calendar.YEAR. So this mean if you want to substract in days, months or years use Calendar.DATE, Calendar.MONTH or Calendar.YEAR respectively.

import java.util.Calendar;

public class CalendarAddExample
{
    public static void main(String[] args)
    {
        Calendar cal = Calendar.getInstance();

        System.out.println("Today : " + cal.getTime());

        // Substract 30 days from the calendar
        cal.add(Calendar.DATE, -30);
        System.out.println("30 days ago: " + cal.getTime());

        // Add 10 months to the calendar
        cal.add(Calendar.MONTH, 10);
        System.out.println("10 months later: " + cal.getTime());

        // Substract 1 year from the calendar
        cal.add(Calendar.YEAR, -1)
        System.out.println("1 year ago: " + cal.getTime());
    }
}

In the code above we want to know what is the date back to 30 days ago.

The sample result of the code is shown below:

Today : Tue Jan 03 06:53:03 ICT 2006
30 days ago: Sun Dec 04 06:53:03 ICT 2005