Calendar: Both set() and roll() don't work for AM_PM time field.
But add() works.

Please run the attached code, the AM_PM won't change until
add() is called.
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import java.util.*;
import java.text.*;

public class Test{
   public static void main(String[] args){
      
      Calendar cal = Calendar.getInstance();
      cal.clear();
      cal.set(2002, 2, 2, 10, 30, 30);
      cal.get(Calendar.YEAR);
      cal.get(Calendar.MONTH);
      cal.get(Calendar.DATE);
      cal.get(Calendar.HOUR);
      cal.get(Calendar.MINUTE);
      cal.get(Calendar.SECOND);
      System.out.println(cal.get(Calendar.AM_PM));//output 0
      
      cal.set(Calendar.AM_PM, 1);
      System.out.println(cal.get(Calendar.AM_PM));//output 0
      
      cal.roll(Calendar.AM_PM, true);
      System.out.println(cal.get(Calendar.AM_PM));//output 0
      
      cal.roll(Calendar.AM_PM, 1);
      System.out.println(cal.get(Calendar.AM_PM));//output 0
      
      cal.add(Calendar.AM_PM, 1);
      System.out.println(cal.get(Calendar.AM_PM));//output 1
      
      cal.add(Calendar.AM_PM, 1);
      System.out.println(cal.get(Calendar.AM_PM));//output 0
      
      cal.add(Calendar.AM_PM, -1);
      System.out.println(cal.get(Calendar.AM_PM));//output 1
   }
}