Created an extra ExInvalidDate.java to handle the exception of invalid date input.

package code;

public class ExInvalidDate extends Exception {

    private String day;

    public String getDay() { return day; }

    public ExInvalidDate() { super("Invalid date."); }

    public ExInvalidDate(String message) { super(message); }
    
}


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In the CmdStartNewDay.java. Changed the public void execute(String[] cmdParts) as follow:

    public void execute(String[] cmdParts) {
        try {

            SystemDate sd = SystemDate.getInstance();
            sOld = sd.toString();
            sNew = cmdParts[1];

            if (!sd.valid(sNew)) throw new ExInvalidDate();

            sd.set(sNew);

            addUndoCommand(this);
            clearRedoList();
            System.out.println("Done.");

        } catch (ExInvalidNewDate e) {
            System.out.println(e.getMessage());

        } catch (ExInvalidDate | ArrayIndexOutOfBoundsException | NumberFormatException e) {
			System.out.println(new ExInvalidDate().getMessage());
		}
    }