question: public static Date daysBefore ( Date dateInst , int numDays ) { if ( ( numDays <= 0 ) || ( dateInst == null ) ) { throw new IllegalArgumentException ( ) ; } final Calendar cal = new GregorianCalendar ( ) ; cal . setTime ( dateInst ) ; cal . add ( Calendar . DAY_OF_YEAR , - numDays ) ; return cal . getTime ( ) ; }
target:  Method to return a Date instance which is exactly numDays days before the point in time designated by the input Date.