If you want to set a default date for a jasperreports parameter, you can enter the following in the default value expression of the parameter.
However, if you wanted the default to be relative from today, for example, 3 days ago, you will need to do go outside of the standard java date libraries (Date, Calendar) because JR parameter does not allow you to have multiple statements. You can write your own utility package or you can use Joda, a replacement library for the JDK date and time library.
To use Joda in iReports, you need to extract the jar file and add it to Options -> Classpath. You may also drop the jar file in iReports' lib directory. You will also need to add this library to your application or report server.
For setting a default date 5 days prior from today, use this:
or this if you want the time part set to the beginning of the day:
or if you want the parameter to be relative to another parameter:
Joda has other methods to get a relative date: minusWeeks, minusMonths, minusYears, plusDays, plusWeeks, plusMonths, plusYears. I think you can also use Groovy to write an expression to get a relative date but that's for another time. It would probably be as simple as new Date() - 5. Post a comment if you came up with a better way.