java.util.Date - Deleting three months from a date?

Here's the plain JDK version, it needs the Calendar class as a helper:

Date referenceDate = new Date();
Calendar c = Calendar.getInstance(); 
c.setTime(referenceDate); 
c.add(Calendar.MONTH, -3);
return c.getTime();

But you should seriously consider using the Joda library, because of various shortcomings of the Date and Calendar classes. With Joda you can do the following:

new DateTime().minusMonths(3).toDate();

Or if you want to subtract from a given date instead of the current:

new DateTime(referenceDate).minusMonths(3).toDate();

Update for Java 8: With Java 8 you can also use the new JSR 310 API (which is inspired by Joda):

LocalDateTime.from(referenceDate.toInstant()).minusMonths(3);

Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
cal.add(Calendar.MONTH, -3);

Set your date using setTime method.

Using Java 8 you can do it like this,

Date d = Date.from(LocalDate.now().minusMonths(3).atStartOfDay(ZoneId.systemDefault()).toInstant());

The LocalDate class has a lot of methods to help you make easy computations about dates like the above,

// Add 2 months
Date d = Date.from(LocalDate.now().plusMonths(2).atStartOfDay(ZoneId.systemDefault()).toInstant());
// Add 5 days
Date d = Date.from(LocalDate.now().plusDays(5).atStartOfDay(ZoneId.systemDefault()).toInstant());
// Minus 1 day and 1 year
Date d = Date.from(LocalDate.now().minusYears(1).minusDays(1).atStartOfDay(ZoneId.systemDefault()).toInstant());

In order to compute time you can use the LocalDateTime class,

// Minus 1 year, minus 1 days, plus 1 hour
Date d = Date.from(LocalDateTime.now().minusYears(1).minusDays(1).plusHours(1).toLocalDate().atStartOfDay(ZoneId.systemDefault()).toInstant());