Time comparation in java these are in string?

import java.util.*;
import java.lang.*;
import java.text.ParseException;
import java.util.Date;
import java.text.SimpleDateFormat;

class Rextester
{  
    public static void main(String args[])
    {
        String startTime = "10:15:20 am";
        String endTime = "12:30:37 pm";

        SimpleDateFormat sdf = new SimpleDateFormat("hh:mm:ss a");
        try {
            
            Date d1 = sdf.parse(startTime);
            
            Date d2 = sdf.parse(endTime);
  
            long elapsed = d2.getTime() - d1.getTime(); 
            
            int hours = (int) Math.floor(elapsed / 3600000);
            
            int minutes = (int) Math.floor((elapsed - hours * 3600000) / 60000);
            
            int seconds = (int) Math.floor((elapsed - hours * 3600000 - minutes * 60000) / 1000);
            
            System.out.format("From %s to %s%n", startTime, endTime);
                                    
            System.out.format("Time elapsed %d milliseconds%n", elapsed);
            
            System.out.format("%d hours %d minutes %d seconds%n", hours, minutes, seconds);


         } catch (ParseException e) {
              e.printStackTrace();
         }
    }
}