Mock new Date() and System.currentTimeMillis() with PowerMockito

SystemTimeTest.java

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

import java.time.Instant;
import java.util.Date;

@RunWith(PowerMockRunner.class)
@PrepareForTest({Client.class, LegacyClass.class})
public class SystemTimeTest {

    private final Date startDate = Date.from(Instant.parse("2010-01-01T00:00:00.00Z"));
    private final Date endDate = Date.from(Instant.parse("2010-12-31T23:59:59.00Z"));

    private final Date fakeNow = Date.from(Instant.parse("2010-12-03T10:15:30.00Z"));

    @Before
    public void init() throws Exception {
        // mock `new Date()`
        PowerMockito.whenNew(Date.class).withNoArguments().thenReturn(fakeNow);
        System.out.println("Fake now: " + fakeNow);
        
        // you can mock other constructors of Date
        // PowerMockito.whenNew(Date.class).withAnyArguments().thenReturn(fakeNow);
        // PowerMockito.whenNew(Date.class).withParameterTypes(long.class).withArguments(ArgumentMatchers.anyLong()).thenReturn(fakeNow);

        // mock System.currentTimeMillis()
        PowerMockito.mockStatic(System.class);
        PowerMockito.when(System.currentTimeMillis()).thenReturn(fakeNow.getTime()); // 2021.03.31 22:42:00 1617205235060L
        System.out.println("Fake currentTimeMillis: " + System.currentTimeMillis());
    }

    @Test
    public void newDate() {
        System.out.println("new Date(): " + new Date());
        Assert.assertTrue(Client.isBetweenDate(startDate, endDate));
    }

    @Test
    public void currentTimeMillis1() {
        System.out.println("currentTimeMillis: " + System.currentTimeMillis());
        Assert.assertTrue(Client.isBetweenMillis(startDate, endDate));
    }

    @Test
    public void currentTimeMillis2() {
        System.out.println("currentTimeMillis: " + System.currentTimeMillis());
        Assert.assertTrue(Client.isBetweenSystem(startDate, endDate));
    }

    @Test
    public void legacyClass() {
        LegacyClass legacyClass = new LegacyClass();
        legacyClass.methodWithNewDate();
        legacyClass.methodWithCurrentTimeMillis();
    }

}

class Client {

    public static boolean isBetweenDate(Date startDate, Date endDate) {
        System.out.println("== Client isBetweenDate ===");
        System.out.println("startDate is " + startDate);
        System.out.println("endDate is " + endDate);
        Date now = new Date();
        System.out.println("Now is " + now);
        return startDate.before(now) && endDate.after(now);
    }

    public static boolean isBetweenMillis(Date startDate, Date endDate) {
        System.out.println("=== Client isBetweenMillis ==");
        System.out.println("startDate is " + startDate.getTime());
        System.out.println("endDate is " + endDate.getTime());
        long now = System.currentTimeMillis();
        System.out.println("Now is " + now);
        return startDate.getTime() < now && endDate.getTime() > now;
    }

    public static boolean isBetweenSystem(Date startDate, Date endDate) {
        System.out.println("=== Client isBetweenSystem ==");
        System.out.println("startDate is " + startDate);
        System.out.println("endDate is " + endDate);
        Date now = Date.from(Instant.ofEpochMilli(System.currentTimeMillis()));
        System.out.println("Now is " + now);
        return startDate.before(now) && endDate.after(now);
    }

}

class LegacyClass {

    public void methodWithNewDate() {
        Date now = new Date();
        System.out.println("LegacyClass new Date() is " + now);
    }

    public void methodWithCurrentTimeMillis() {
        long now = System.currentTimeMillis();
        System.out.println("LegacyClass System.currentTimeMillis() is " + now);
    }

}