no test is available in visual studio

This error showed up after adding a new class to a test project.

using Microsoft.VisualStudio.TestTools.UnitTesting;
using DateTime;

namespace UnitTestDateTime
{
    [TestClass]
    class UnitTestDate
    {
        [TestMethod]
        public void TestDayOfTheWeek()
        {
            Date date = new Date(1, 27, 2019);
            Assert.AreEqual("Sunday", date.DayOfTheWeek());
        }
    }
}

[1/27/2019 9:18:19 AM Informational] ------ Discover test started ------
[1/27/2019 9:18:23 AM Warning] [MSTest][Discovery][C:\Users\tjmad\source\repos\DateTime\UnitTestDateTime\bin\Debug\netcoreapp2.1\UnitTestDateTime.dll] UTA001: TestClass attribute defined on non-public class UnitTestDateTime.UnitTestDate
[1/27/2019 9:18:23 AM Warning] No test is available in C:\Users\tjmad\source\repos\DateTime\UnitTestDateTime\bin\Debug\netcoreapp2.1\UnitTestDateTime.dll. Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again.
[1/27/2019 9:18:24 AM Warning] No test is available in C:\Users\tjmad\source\repos\DateTime\UnitTestDateTime\UnitTestDateTime.csproj. Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again.
[1/27/2019 9:18:24 AM Informational] ========== Discover test finished: 0 found (0:00:05.451639) ==========
[1/27/2019 9:18:24 AM Informational] ------ Run test started ------
[1/27/2019 9:18:26 AM Warning] [MSTest][Discovery][C:\Users\tjmad\source\repos\DateTime\UnitTestDateTime\bin\Debug\netcoreapp2.1\UnitTestDateTime.dll] UTA001: TestClass attribute defined on non-public class UnitTestDateTime.UnitTestDate
[1/27/2019 9:18:26 AM Warning] No test is available in C:\Users\tjmad\source\repos\DateTime\UnitTestDateTime\bin\Debug\netcoreapp2.1\UnitTestDateTime.dll. Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again.
[1/27/2019 9:18:27 AM Warning] No test is available in C:\Users\tjmad\source\repos\DateTime\DateTime\DateTime.csproj C:\Users\tjmad\source\repos\DateTime\UnitTestDateTime\UnitTestDateTime.csproj. Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again.
[1/27/2019 9:18:27 AM Informational] ========== Run test finished: 0 run (0:00:03.1466992) ==========

The solution was simple. The class needed to be public.

using Microsoft.VisualStudio.TestTools.UnitTesting;
using DateTime;

namespace UnitTestDateTime
{
    [TestClass]
    public class UnitTestDate
    {
        [TestMethod]
        public void TestDayOfTheWeek()
        {
            Date date = new Date(1, 27, 2019);
            Assert.AreEqual("Sunday", date.DayOfTheWeek());

        }

    }
}