diff --git a/dotnet/SafelyYou.slnx b/dotnet/SafelyYou.slnx
index f93bb03..1cbfb08 100644
--- a/dotnet/SafelyYou.slnx
+++ b/dotnet/SafelyYou.slnx
@@ -1,3 +1,4 @@
 <Solution>
   <Project Path="SafelyYou/SafelyYou.csproj" />
+  <Project Path="SafelyYouTests/SafelyYouTests.csproj" />
 </Solution>
diff --git a/dotnet/SafelyYouTests/DeviceHeartBeatHistoryTests.cs b/dotnet/SafelyYouTests/DeviceHeartBeatHistoryTests.cs
new file mode 100644
index 0000000..d989f02
--- /dev/null
+++ b/dotnet/SafelyYouTests/DeviceHeartBeatHistoryTests.cs
@@ -0,0 +1,111 @@
+using SafelyYou;
+
+namespace SafelyYouTests;
+
+public class DeviceHeartBeatHistoryTests
+{
+    private static readonly DateTimeOffset T0 = new(2024, 1, 1, 12, 0, 0, TimeSpan.Zero);
+    private static readonly DateTimeOffset T1 = T0.AddMinutes(1);
+    private static readonly DateTimeOffset T2 = T0.AddMinutes(2);
+
+    [Test]
+    public void Add_FirstHeartbeat_SetsFirstLastAndCount()
+    {
+        var history = new DeviceHeartBeatHistory();
+
+        var result = history.Add(T0);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(result, Is.EqualTo(DeviceHeartBeatHistory.AddResult.Added));
+            Assert.That(history.NumberOfHeartBeats, Is.EqualTo(1));
+            Assert.That(history.FirstHeartBeat, Is.EqualTo(T0));
+            Assert.That(history.LastHeartBeat, Is.EqualTo(T0));
+        });
+    }
+
+    [Test]
+    public void Add_LaterHeartbeat_UpdatesLastAndIncrementsCount()
+    {
+        var history = new DeviceHeartBeatHistory();
+        history.Add(T0);
+
+        var result = history.Add(T1);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(result, Is.EqualTo(DeviceHeartBeatHistory.AddResult.Added));
+            Assert.That(history.NumberOfHeartBeats, Is.EqualTo(2));
+            Assert.That(history.FirstHeartBeat, Is.EqualTo(T0));
+            Assert.That(history.LastHeartBeat, Is.EqualTo(T1));
+        });
+    }
+
+    [Test]
+    public void Add_SameTimestampAsLast_IsAccepted()
+    {
+        var history = new DeviceHeartBeatHistory();
+        history.Add(T0);
+
+        var result = history.Add(T0);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(result, Is.EqualTo(DeviceHeartBeatHistory.AddResult.Added));
+            Assert.That(history.NumberOfHeartBeats, Is.EqualTo(2));
+            Assert.That(history.FirstHeartBeat, Is.EqualTo(T0));
+            Assert.That(history.LastHeartBeat, Is.EqualTo(T0));
+        });
+    }
+
+    [Test]
+    public void Add_EarlierThanLast_ReturnsOutOfSequenceAndDoesNotMutate()
+    {
+        var history = new DeviceHeartBeatHistory();
+        history.Add(T1);
+
+        var result = history.Add(T0);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(result, Is.EqualTo(DeviceHeartBeatHistory.AddResult.OutOfSequence));
+            Assert.That(history.NumberOfHeartBeats, Is.EqualTo(1));
+            Assert.That(history.FirstHeartBeat, Is.EqualTo(T1));
+            Assert.That(history.LastHeartBeat, Is.EqualTo(T1));
+        });
+    }
+
+    [Test]
+    public void Add_MultipleInOrder_KeepsFirstAndAdvancesLast()
+    {
+        var history = new DeviceHeartBeatHistory();
+
+        Assert.That(history.Add(T0), Is.EqualTo(DeviceHeartBeatHistory.AddResult.Added));
+        Assert.That(history.Add(T1), Is.EqualTo(DeviceHeartBeatHistory.AddResult.Added));
+        Assert.That(history.Add(T2), Is.EqualTo(DeviceHeartBeatHistory.AddResult.Added));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(history.NumberOfHeartBeats, Is.EqualTo(3));
+            Assert.That(history.FirstHeartBeat, Is.EqualTo(T0));
+            Assert.That(history.LastHeartBeat, Is.EqualTo(T2));
+        });
+    }
+
+    [Test]
+    public void Add_WithGap_StillCountsHeartbeat()
+    {
+        var history = new DeviceHeartBeatHistory();
+        history.Add(T0);
+
+        var muchLater = T0.AddHours(1);
+        var result = history.Add(muchLater);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(result, Is.EqualTo(DeviceHeartBeatHistory.AddResult.Added));
+            Assert.That(history.NumberOfHeartBeats, Is.EqualTo(2));
+            Assert.That(history.LastHeartBeat, Is.EqualTo(muchLater));
+        });
+    }
+}
diff --git a/dotnet/SafelyYouTests/HeartBeatHistoriesUptimeTests.cs b/dotnet/SafelyYouTests/HeartBeatHistoriesUptimeTests.cs
new file mode 100644
index 0000000..f97f1a8
--- /dev/null
+++ b/dotnet/SafelyYouTests/HeartBeatHistoriesUptimeTests.cs
@@ -0,0 +1,89 @@
+using SafelyYou;
+
+namespace SafelyYouTests;
+
+public class HeartBeatHistoriesUptimeTests
+{
+    private static readonly DeviceId Device = new("device-1");
+    private static readonly DateTimeOffset T0 = new(2024, 1, 1, 12, 0, 0, TimeSpan.Zero);
+
+    [Test]
+    public void Uptime_UnknownDevice_ReturnsZero()
+    {
+        var histories = new HeartBeatHistories();
+
+        Assert.That(histories.Uptime(Device), Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Uptime_SingleHeartbeat_ReturnsZero()
+    {
+        var histories = new HeartBeatHistories();
+        histories.Add(Device, T0);
+
+        // first == last => TotalMinutes is 0, guarded to return 0
+        Assert.That(histories.Uptime(Device), Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Uptime_TwoHeartbeatsOneMinuteApart_ReturnsTwoHundred()
+    {
+        var histories = new HeartBeatHistories();
+        histories.Add(Device, T0);
+        histories.Add(Device, T0.AddMinutes(1));
+
+        // (2 beats / 1 minute) * 100
+        Assert.That(histories.Uptime(Device), Is.EqualTo(200).Within(0.001));
+    }
+
+    [Test]
+    public void Uptime_TwoHeartbeatsOverTwoMinutes_ReturnsOneHundred()
+    {
+        var histories = new HeartBeatHistories();
+        histories.Add(Device, T0);
+        histories.Add(Device, T0.AddMinutes(2));
+
+        // (2 beats / 2 minutes) * 100
+        Assert.That(histories.Uptime(Device), Is.EqualTo(100).Within(0.001));
+    }
+
+    [Test]
+    public void Uptime_ThreeHeartbeatsOverTwoMinutes_ReturnsOneHundredFifty()
+    {
+        var histories = new HeartBeatHistories();
+        histories.Add(Device, T0);
+        histories.Add(Device, T0.AddMinutes(1));
+        histories.Add(Device, T0.AddMinutes(2));
+
+        // (3 beats / 2 minutes) * 100
+        Assert.That(histories.Uptime(Device), Is.EqualTo(150).Within(0.001));
+    }
+
+    [Test]
+    public void Uptime_IsPerDevice()
+    {
+        var other = new DeviceId("device-2");
+        var histories = new HeartBeatHistories();
+        histories.Add(Device, T0);
+        histories.Add(Device, T0.AddMinutes(1));
+        histories.Add(other, T0);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(histories.Uptime(Device), Is.EqualTo(200).Within(0.001));
+            Assert.That(histories.Uptime(other), Is.EqualTo(0));
+        });
+    }
+
+    [Test]
+    public void Uptime_OutOfSequenceHeartbeat_DoesNotAffectCalculation()
+    {
+        var histories = new HeartBeatHistories();
+        histories.Add(Device, T0.AddMinutes(1));
+        histories.Add(Device, T0); // rejected
+        histories.Add(Device, T0.AddMinutes(3));
+
+        // still 2 beats over 2 minutes
+        Assert.That(histories.Uptime(Device), Is.EqualTo(100).Within(0.001));
+    }
+}
diff --git a/dotnet/SafelyYouTests/SafelyYouTests.csproj b/dotnet/SafelyYouTests/SafelyYouTests.csproj
new file mode 100644
index 0000000..7009be5
--- /dev/null
+++ b/dotnet/SafelyYouTests/SafelyYouTests.csproj
@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <LangVersion>latest</LangVersion>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+        <IsPackable>false</IsPackable>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="coverlet.collector" Version="6.0.4"/>
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.0"/>
+        <PackageReference Include="NUnit" Version="4.3.2"/>
+        <PackageReference Include="NUnit.Analyzers" Version="4.7.0"/>
+        <PackageReference Include="NUnit3TestAdapter" Version="5.0.0"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <Using Include="NUnit.Framework"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\SafelyYou\SafelyYou.csproj"/>
+    </ItemGroup>
+
+</Project>