How do I force a start date to be at least 30 days before an end date when registering?

Question

Right now, I have a method that checks that the start date is before the end date. How would I make sure that the start is a minimum of 30 days before the end date?

The code looks like this:

public static boolean CheckDates(String start_date, String end_date) {

    SimpleDateFormat dfDate = new SimpleDateFormat("yyyy-MM-dd");

    boolean b = false;

    try {
        if (dfDate.parse(start_date).before(dfDate.parse(end_date))) {
            b = true;  // If start date is before end date.
        } else if (dfDate.parse(start_date).equals(dfDate.parse(end_date))) {
            b = false;  // If two dates are equal.
        } else {
            b = false; // If start date is after the end date.
        }
    } catch (ParseException e) {
        e.printStackTrace();
    }

    return b;
}

Answer 1

LocalDate::minusDays

If you were using the newer java.time library, you get minus… methods such as LocalDate::minusDays.

LocalDate ld1 = LocalDate.now();            
LocalDate ld2 = ld1.minusDays(31);
if ( ld2.isBefore( ld1.minusDays(30) ) ) {
    // Yes, ld2 is earlier than 30 days before ld1 ...
}

Edit: The OP asked for different logic than my answer. Here's my response to the request in the comments.

LocalDate ld1 = LocalDate.parse("2020-06-14");            
LocalDate ld2 = LocalDate.parse("2020-07-01"); 
if ( ld2.isBefore( ld1.plusDays(30) ) ) {
    // Warning, ld2 is less than 30 days away from ld1 ...
}