SystemClockTimer test

DesktopClock.Tests/SystemClockTests.cs

@@ -0,0 +1,47 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace DesktopClock.Tests;
+
+public class SystemClockTimerTests
+{
+    private readonly SystemClockTimer _timer;
+
+    public SystemClockTimerTests()
+    {
+        _timer?.Dispose();
+        _timer = new SystemClockTimer();
+    }
+
+    [Theory]
+    [InlineData(3)]
+    public async Task ShouldTickEverySecondAccurately(int seconds)
+    {
+        // Ensure the timer is started at an unclean time to test accuracy.
+        await Task.Delay(1000 - DateTimeOffset.Now.Millisecond + 234);
+
+        Assert.NotInRange(DateTimeOffset.Now.Millisecond, 0, 100);
+
+        var tickTimes = new List<DateTimeOffset>();
+
+        _timer.SecondChanged += (sender, args) =>
+        {
+            tickTimes.Add(DateTimeOffset.Now);
+        };
+
+        _timer.Start();
+
+        await Task.Delay(TimeSpan.FromSeconds(seconds));
+
+        _timer.Stop();
+
+        Assert.Equal(seconds, tickTimes.Count);
+
+        // Check that each tick is close to the exact second.
+        foreach (var tickTime in tickTimes)
+        {
+            Assert.InRange(tickTime.Millisecond, 0, 100);
+        }
+    }
+}