The below code gives the last date of the respective month. The parameter "appln_date" should be passed in DDMMyyyy formate and the parameter "appln_Lastdate" consists the last date of the month. You can find the last day from the date value "appln_Lastdate"

DateTimeUtils dtu = tools.getDateTimeUtils();

String appln_date = tools.getParamValue("appln_date") + "T000000.000 GMT";

java.util.Calendar startCal = null;

startCal = java.util.Calendar.getInstance();

Date dStart = dtu.parseDateTimeString(appln_date);

startCal.setTime(dStart);

startCal.add(java.util.Calendar.MONTH, 1);

startCal.set(java.util.Calendar.DAY_OF_MONTH, 0);

StringBuffer sbuf = new StringBuffer();

sbuf.append(PRDateFormat.formatInternalDateTime(startCal.getTime()));

String LastDay = sbuf.toString();

LastDay = pega_rules_string.whatComesBeforeFirst(LastDay,'T');

tools.putParamValue("appln_Lastdate",LastDay);