freeze/unfreeze operations date in java / virtual clock pattern

Clock.java

import java.util.Calendar;
import java.util.Date;

public final class Clock {

    private static boolean isFrozen;

    private static Date timeSet;

    private Clock() {
    }

    public static synchronized void freeze() {
        isFrozen = true;
    }

    public static synchronized void freeze(Date date) {
        freeze();
        timeSet = date;
    }

    public static synchronized void unfreeze() {
        isFrozen = false;
        timeSet = null;
    }

    public static Date getTime() {
        Calendar calendar = Calendar.getInstance();
        if (isFrozen) {
            if (timeSet == null) {
                timeSet = calendar.getTime();
            }
            return timeSet;
        }
        return calendar.getTime();
    }

}