diff --git a/src/SIL.Harmony.Tests/Core/HybridDateTimeProviderTests.cs b/src/SIL.Harmony.Tests/Core/HybridDateTimeProviderTests.cs
new file mode 100644
index 0000000..0b952f9
--- /dev/null
+++ b/src/SIL.Harmony.Tests/Core/HybridDateTimeProviderTests.cs
@@ -0,0 +1,101 @@
+namespace SIL.Harmony.Tests.Core;
+
+/// <summary>
+/// Exercises the real <see cref="HybridDateTimeProvider"/> clock logic. The rest of the suite injects a
+/// MockTimeProvider that stubs this behaviour out, so without these tests the hybrid-clock guarantees
+/// (monotonic timestamps when the wall clock goes backward, advancing past synced commits) have no coverage.
+/// </summary>
+public class HybridDateTimeProviderTests
+{
+    private sealed class SettableTimeProvider : TimeProvider
+    {
+        public DateTimeOffset Now { get; set; }
+        public override DateTimeOffset GetUtcNow() => Now;
+    }
+
+    private static readonly DateTimeOffset _baseTime = new(2000, 1, 1, 0, 0, 0, TimeSpan.Zero);
+
+    private static (HybridDateTimeProvider provider, SettableTimeProvider clock) NewProvider(HybridDateTime lastDateTime)
+    {
+        var clock = new SettableTimeProvider { Now = _baseTime };
+        return (new HybridDateTimeProvider(clock, lastDateTime), clock);
+    }
+
+    [Fact]
+    public void GetDateTime_UsesWallClockWhenItIsAheadOfLastTime()
+    {
+        var (provider, clock) = NewProvider(new HybridDateTime(_baseTime.AddHours(-1), 5));
+        clock.Now = _baseTime;
+
+        var result = provider.GetDateTime();
+
+        result.DateTime.Should().Be(_baseTime);
+        result.Counter.Should().Be(0, "a forward clock resets the counter");
+    }
+
+    [Fact]
+    public void GetDateTime_WhenClockGoesBackward_ReusesLastTimeAndIncrementsCounter()
+    {
+        //lastDateTime is in the future relative to the wall clock (clock was reset backwards)
+        var (provider, clock) = NewProvider(new HybridDateTime(_baseTime.AddHours(1), 0));
+        clock.Now = _baseTime;
+
+        var first = provider.GetDateTime();
+        first.DateTime.Should().Be(_baseTime.AddHours(1), "the newer timestamp is kept, not the regressed wall clock");
+        first.Counter.Should().Be(1);
+
+        //still behind, so the counter keeps climbing to stay monotonic
+        var second = provider.GetDateTime();
+        second.DateTime.Should().Be(_baseTime.AddHours(1));
+        second.Counter.Should().Be(2);
+
+        (second > first).Should().BeTrue();
+    }
+
+    [Fact]
+    public void GetDateTime_WhenClockEqualsLastTime_IncrementsCounter()
+    {
+        var (provider, clock) = NewProvider(new HybridDateTime(_baseTime, 0));
+        clock.Now = _baseTime;
+
+        var result = provider.GetDateTime();
+
+        result.DateTime.Should().Be(_baseTime);
+        result.Counter.Should().Be(1, "equal timestamps must still advance via the counter");
+    }
+
+    [Fact]
+    public void TakeLatestTime_AdvancesToNewestReceivedTime()
+    {
+        var (provider, clock) = NewProvider(new HybridDateTime(_baseTime, 0));
+
+        provider.TakeLatestTime([
+            new HybridDateTime(_baseTime.AddHours(-1), 9),
+            new HybridDateTime(_baseTime.AddHours(2), 3),
+            new HybridDateTime(_baseTime.AddHours(1), 0),
+        ]);
+
+        //next local write must sort after the newest received commit even though the wall clock is older
+        clock.Now = _baseTime;
+        var next = provider.GetDateTime();
+        next.DateTime.Should().Be(_baseTime.AddHours(2));
+        next.Counter.Should().Be(4);
+    }
+
+    [Fact]
+    public void TakeLatestTime_IgnoresTimesOlderThanCurrent()
+    {
+        var (provider, clock) = NewProvider(new HybridDateTime(_baseTime.AddHours(5), 2));
+
+        provider.TakeLatestTime([
+            new HybridDateTime(_baseTime, 0),
+            new HybridDateTime(_baseTime.AddHours(1), 0),
+        ]);
+
+        //current time was newer than everything received, so it is unchanged
+        clock.Now = _baseTime;
+        var next = provider.GetDateTime();
+        next.DateTime.Should().Be(_baseTime.AddHours(5));
+        next.Counter.Should().Be(3);
+    }
+}
diff --git a/src/SIL.Harmony.Tests/Core/HybridDateTimeTests.cs b/src/SIL.Harmony.Tests/Core/HybridDateTimeTests.cs
index cb75ae3..68477ae 100644
--- a/src/SIL.Harmony.Tests/Core/HybridDateTimeTests.cs
+++ b/src/SIL.Harmony.Tests/Core/HybridDateTimeTests.cs
@@ -72,4 +72,46 @@ public void CompareTo_ReturnsOneWhenThisIsGreaterThanOther()
         var result = dateTime.CompareTo(otherDateTime);
         result.Should().Be(1);
     }
+
+    [Fact]
+    public void CompareTo_BreaksTiesByCounterWhenDateTimeIsEqual()
+    {
+        var instant = new DateTimeOffset(2000, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        var lower = new HybridDateTime(instant, 1);
+        var higher = new HybridDateTime(instant, 2);
+
+        lower.CompareTo(higher).Should().BeLessThan(0);
+        higher.CompareTo(lower).Should().BeGreaterThan(0);
+    }
+
+    [Theory]
+    //same instant, ordered by counter
+    [InlineData("2000-01-01", 1, "2000-01-01", 2, true)]
+    [InlineData("2000-01-01", 2, "2000-01-01", 1, false)]
+    //different instant, counter is irrelevant
+    [InlineData("2000-01-01", 9, "2000-01-02", 0, true)]
+    [InlineData("2000-01-02", 0, "2000-01-01", 9, false)]
+    public void Operators_OrderByDateTimeThenCounter(string leftDate, long leftCounter, string rightDate, long rightCounter, bool leftIsSmaller)
+    {
+        var left = new HybridDateTime(DateTimeOffset.Parse(leftDate + "T00:00:00Z"), leftCounter);
+        var right = new HybridDateTime(DateTimeOffset.Parse(rightDate + "T00:00:00Z"), rightCounter);
+
+        (left < right).Should().Be(leftIsSmaller);
+        (left <= right).Should().Be(leftIsSmaller);
+        (left > right).Should().Be(!leftIsSmaller);
+        (left >= right).Should().Be(!leftIsSmaller);
+    }
+
+    [Fact]
+    public void Operators_LessOrEqualAndGreaterOrEqual_TrueWhenEqual()
+    {
+        var instant = new DateTimeOffset(2000, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        var left = new HybridDateTime(instant, 3);
+        var right = new HybridDateTime(instant, 3);
+
+        (left <= right).Should().BeTrue();
+        (left >= right).Should().BeTrue();
+        (left < right).Should().BeFalse();
+        (left > right).Should().BeFalse();
+    }
 }
diff --git a/src/SIL.Harmony.Tests/SyncTests.cs b/src/SIL.Harmony.Tests/SyncTests.cs
index a3d7d66..92ed6e6 100644
--- a/src/SIL.Harmony.Tests/SyncTests.cs
+++ b/src/SIL.Harmony.Tests/SyncTests.cs
@@ -149,4 +149,77 @@ public async Task CanSyncCommitsWithMoreEntitiesThanTheSqliteParameterLimit()
 
         _client1.DbContext.Set<Word>().Should().HaveCount(totalEntityCount);
     }
+
+    [Fact]
+    public async Task ConcurrentEditsToSameEntity_ConvergeToSameValueOnBothReplicas()
+    {
+        var entityId = Guid.NewGuid();
+        //both clients edit the same entity at the same instant (same DateTime, counter 0): a genuine tie
+        //that must be broken deterministically so both replicas pick the same winner.
+        var sharedDate = new DateTime(2005, 1, 1);
+        _client1.SetCurrentDate(sharedDate);
+        _client2.SetCurrentDate(sharedDate);
+        await _client1.WriteNextChange(_client1.SetWord(entityId, "from client1"));
+        await _client2.WriteNextChange(_client2.SetWord(entityId, "from client2"));
+
+        await _client1.DataModel.SyncWith(_client2.DataModel);
+
+        var client1Value = (await _client1.DataModel.GetLatest<Word>(entityId))!.Text;
+        var client2Value = (await _client2.DataModel.GetLatest<Word>(entityId))!.Text;
+        client1Value.Should().Be(client2Value, "concurrent edits must converge to a single deterministic winner");
+        client1Value.Should().BeOneOf("from client1", "from client2");
+
+        var client1Snapshot = await _client1.DataModel.GetProjectSnapshot();
+        var client2Snapshot = await _client2.DataModel.GetProjectSnapshot();
+        client1Snapshot.LastCommitHash.Should().Be(client2Snapshot.LastCommitHash);
+    }
+
+    [Fact]
+    public async Task ConcurrentEditsToSameEntity_LaterTimestampWins()
+    {
+        var entityId = Guid.NewGuid();
+        _client1.SetCurrentDate(new DateTime(2005, 1, 1));
+        _client2.SetCurrentDate(new DateTime(2005, 6, 1)); //strictly later
+        await _client1.WriteNextChange(_client1.SetWord(entityId, "early"));
+        await _client2.WriteNextChange(_client2.SetWord(entityId, "late"));
+
+        await _client1.DataModel.SyncWith(_client2.DataModel);
+
+        (await _client1.DataModel.GetLatest<Word>(entityId))!.Text.Should().Be("late");
+        (await _client2.DataModel.GetLatest<Word>(entityId))!.Text.Should().Be("late");
+    }
+
+    [Fact]
+    public async Task SyncWith_CalledTwice_SecondSyncTransfersNothing()
+    {
+        await _client1.WriteNextChange(_client1.SetWord(Guid.NewGuid(), "entity1"));
+        await _client2.WriteNextChange(_client2.SetWord(Guid.NewGuid(), "entity2"));
+
+        var firstSync = await _client1.DataModel.SyncWith(_client2.DataModel);
+        firstSync.IsSynced.Should().BeTrue();
+        (firstSync.MissingFromLocal.Length + firstSync.MissingFromRemote.Length).Should().BeGreaterThan(0);
+
+        var secondSync = await _client1.DataModel.SyncWith(_client2.DataModel);
+        secondSync.IsSynced.Should().BeTrue();
+        secondSync.MissingFromLocal.Should().BeEmpty("both replicas are already converged");
+        secondSync.MissingFromRemote.Should().BeEmpty("both replicas are already converged");
+
+        var client1Snapshot = await _client1.DataModel.GetProjectSnapshot();
+        var client2Snapshot = await _client2.DataModel.GetProjectSnapshot();
+        client1Snapshot.LastCommitHash.Should().Be(client2Snapshot.LastCommitHash);
+    }
+
+    [Fact]
+    public async Task SyncWith_WhenRemoteShouldNotSync_ReturnsNotSyncedAndTransfersNothing()
+    {
+        await _client1.WriteNextChange(_client1.SetWord(Guid.NewGuid(), "entity1"));
+        var beforeSyncCount = _client1.DbContext.Commits.Count();
+
+        var results = await _client1.DataModel.SyncWith(NullSyncable.Instance);
+
+        results.IsSynced.Should().BeFalse();
+        results.MissingFromLocal.Should().BeEmpty();
+        results.MissingFromRemote.Should().BeEmpty();
+        _client1.DbContext.Commits.Count().Should().Be(beforeSyncCount, "a declined sync must not change local state");
+    }
 }