How do you show the number of days per month?

Question

How do you show the number of days per month?
I am working on a budget.  I want to know how much to amortize each month to fund an investment.  Income comes monthly, but expenses leave in days, or weeks.  The number of days and weeks in months vary. 
I want to figure out income and expenses per day, per month and per year, so I know how much can be invested each month, week or day.  For a start I would like a formula that shows how many days are in each month?

Answer 1

thanks..
I solve my problem as
public class MyExample {
    public static void main(String a[]) {
        String stdate = "2009-03-01";
        java.sql.Date currentDate = new java.sql.Date(System.currentTimeMillis());
        java.sql.Date preDate = java.sql.Date.valueOf(stdate);
        System.out.println(currentDate);
        System.out.println(preDate);
//        int dateCom = preDate.compareTo(currentDate);
//        System.out.println(dateCom);
        long diff = currentDate.getTime() - preDate.getTime();
        int days = (int) Math.floor(diff / (24 * 60 * 60 * 1000));
         System.out.println(days);
}