Date math

[orchid]

Am new to java and having an issue trying to do some date math.
I made this test code and it returns a date difference of 24 days which is what I wanted. My question is this the correct way to subtract dates or should I be doing something more sophisticated?

Code:

private void playWithDates()
    {
    Calendar aDay = new GregorianCalendar();
    aDay.set(2007, Calendar.JANUARY, 25, 0, 0, 0);

    Calendar newYearsDay = new GregorianCalendar();
    newYearsDay.set(2007, Calendar.JANUARY, 1, 0, 0, 0);

    long nyd=newYearsDay.getTimeInMillis();
    long a=aDay.getTimeInMillis();
    long diff =a-nyd;
   
    System.out.println(diff/1000/60/60/24);
    }

output is: 24
thx

[derrickD]

Well that does work so I'd say if it meets your needs then go for it.
There is a very large and involved discussion on date math that you might want to read here:
java date math

[goldhouse]

Calendar class helps you to achieve this with the help roll method,
The description is as below from javadoc of sun

Code:

roll

public void roll(int field,
                 int amount)

    Add to field a signed amount without changing larger fields. A negative roll amount means to subtract from field without changing larger fields.

    Example: Consider a GregorianCalendar originally set to August 31, 1999. Calling roll(Calendar.MONTH, 8) sets the calendar to April 30, 1999. Using a GregorianCalendar, the DAY_OF_MONTH field cannot be 31 in the month April. DAY_OF_MONTH is set to the closest possible value, 30. The YEAR field maintains the value of 1999 because it is a larger field than MONTH.

    Example: Consider a GregorianCalendar originally set to Sunday June 6, 1999. Calling roll(Calendar.WEEK_OF_MONTH, -1) sets the calendar to Tuesday June 1, 1999, whereas calling add(Calendar.WEEK_OF_MONTH, -1) sets the calendar to Sunday May 30, 1999. This is because the roll rule imposes an additional constraint: The MONTH must not change when the WEEK_OF_MONTH is rolled. Taken together with add rule 1, the resultant date must be between Tuesday June 1 and Saturday June 5. According to add rule 2, the DAY_OF_WEEK, an invariant when changing the WEEK_OF_MONTH, is set to Tuesday, the closest possible value to Sunday (where Sunday is the first day of the week).

    Overrides:
        roll in class Calendar

    Parameters:
        field - the time field.
        amount - the signed amount to add to field.
    Since:
        1.2
    See Also:
        add(int, int), Calendar.set(int, int)