Question

How to check whether the given date is earlier than a month ago? What is the fastest algorithm? I have to take into account that different months have different numbers of days.

Answer 1

Updated to Java 8

The class LocalDate class can be used:

LocalDate aDate = LocalDate.parse("2017-01-01");
return aDate.isBefore( LocalDate.now().minusMonths(1));

For previous versions, the Calendar class would work.

Calendar calendar = Calendar.getInstance();
calendar.add( Calendar.MONTH ,  -1 );
return aDate.compareTo( calendar.getTime() ) < 0;

Sample code:

import static java.lang.System.out;
import java.time.LocalDate;

public class Sample {
    public static void main( String [] args ) {
        LocalDate aMonthAgo = LocalDate.now().minusMonths(1);
        out.println( LocalDate.parse("2009-12-16").isBefore(aMonthAgo));
        out.println( LocalDate.now().isBefore(aMonthAgo));
        out.println( LocalDate.parse("2017-12-24").isBefore(aMonthAgo));
    }
}

Prints

true
false
false