First attempt, well almost, at using FakeItEasy

fakeiteasy.cs

[Test]
public void Timestamp_is_transfered_to_journalentry()
{
    JournalEntry entry = null;

    var fake = A.Fake<IJournalWriter>();
    
    A.CallTo(() => fake.Write(A<JournalEntry>._))
        .Invokes((JournalEntry je) =>
        {
            entry = je;
        });
    
    var command = new SetTimeCommand();
    command.Timestamp = DateTime.Now;
    var target = new JournalAppender(0, fake);
    target.Append(command);

    Assert.IsNotNull(entry);
    Assert.AreEqual(command.Timestamp, entry.Created);
}

Looks good. An alternative may be

Test]
public void Timestamp_is_transfered_to_journalentry()
{
    var fake = A.Fake<IJournalWriter>();

    var command = new SetTimeCommand();
    command.Timestamp = DateTime.Now;
    var target = new JournalAppender(0, fake);
    target.Append(command);

    A.CallTo(() => fake.Write(A<JournalEntry>.That.Matches(je => je != null &&
                                                                 je.Created == command.Timestamp)))
                       .MustHaveHappened();
}

Of course, I am not a compiler, so I may be slightly off.
It at least partly comes down to whether you prefer NUnit's messages or FakeItEasy's.
In this case, though, I think the MustHaveHappened is more intention-revealing than the Invokes and the extra entry variable.