I always forget how to do these things so I thought I should write it down.

Changing Dates:

Here is an example of getting the date for seven days ago from now.

    Calendar sevenDaysAgo = Calendar.getInstance();
    sevenDaysAgo.add(Calendar.DATE, -7);
    sevenDaysAgo.set(Calendar.HOUR_OF_DAY, 0);
    sevenDaysAgo.set(Calendar.MINUTE, 0);
    sevenDaysAgo.set(Calendar.SECOND, 0);
    sevenDaysAgo.set(Calendar.MILLISECOND, 0);

Formatting Dates:

Here is the simple way to format a date and time.

    DateFormat.getDateTimeInstance().format(sevenDaysAgo.getTime()))

The output is:

    May 21, 2008 12:00:00 AM