diff --git a/.github/workflows/csharp.yml b/.github/workflows/csharp.yml
index cb785e0..6333c28 100644
--- a/.github/workflows/csharp.yml
+++ b/.github/workflows/csharp.yml
@@ -7,20 +7,35 @@ on:
   pull_request:
 
 jobs:
-  test:
+  test-fx:
+    runs-on: windows-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+      - name: Set up dotnet ${{ matrix.version }}.
+        uses: actions/setup-dotnet@v5
+        with:
+          dotnet-version: "10.0.x"
+      - name: Build
+        run: dotnet build --framework net48
+      - name: Run test suite
+        run: dotnet test --framework net48
+  test-net:
+    strategy:
+      matrix:
+        version:
+          - "8.0"
+          - "9.0"
+          - "10.0"
     runs-on: ubuntu-latest
     name: Test dotnet
     steps:
       - uses: actions/checkout@v6
-      - name: Set up dotnet 8.
-        uses: actions/setup-dotnet@v5
-        with:
-          dotnet-version: '8.0.x'
-      - name: Set up dotnet 10.
+      - name: Set up dotnet ${{ matrix.version }}.
         uses: actions/setup-dotnet@v5
         with:
-          dotnet-version: '10.0.x'
+          dotnet-version: "${{ matrix.version }}.x"
       - name: Build
-        run: dotnet build
+        run: dotnet build --framework net${{ matrix.version }}
       - name: Run test suite
-        run: dotnet test
+        run: dotnet test --framework net${{ matrix.version }}
diff --git a/Personnummer.Tests/CoordinationNumberTests.cs b/Personnummer.Tests/CoordinationNumberTests.cs
index 9566735..81235f4 100644
--- a/Personnummer.Tests/CoordinationNumberTests.cs
+++ b/Personnummer.Tests/CoordinationNumberTests.cs
@@ -196,7 +196,7 @@ public void TestMaleFemale(PersonnummerData ssn)
         [ClassData(typeof(ValidCnDataProvider))]
         public void TestSeparator(PersonnummerData ssn)
         {
-            string sep = ssn.SeparatedFormat.Contains('+') ? "+" : "-";
+            string sep = ssn.SeparatedFormat.Contains("+") ? "+" : "-";
             Assert.Equal(sep, Personnummer.Parse(ssn.LongFormat, new Personnummer.Options { AllowCoordinationNumber = true }).Separator);
             Assert.Equal(sep, Personnummer.Parse(ssn.SeparatedLong, new Personnummer.Options { AllowCoordinationNumber = true }).Separator);
             Assert.Equal(sep, Personnummer.Parse(ssn.SeparatedFormat, new Personnummer.Options { AllowCoordinationNumber = true }).Separator);
@@ -227,7 +227,7 @@ public void TestIgnoreSeparatorWhenFormatting(PersonnummerData ssn)
         [ClassData(typeof(ValidCnDataProvider))]
         public void TestDateCn(PersonnummerData data)
         {
-            var expect = data.LongFormat[..4];
+            var expect = data.LongFormat.Substring(0, 4);
             expect += data.LongFormat.Substring(4, 2);
             expect += (int.Parse(data.LongFormat.Substring(6, 2)) - 60).ToString("00");
 
diff --git a/Personnummer.Tests/Personnummer.Tests.csproj b/Personnummer.Tests/Personnummer.Tests.csproj
index 14abb66..038aa0d 100644
--- a/Personnummer.Tests/Personnummer.Tests.csproj
+++ b/Personnummer.Tests/Personnummer.Tests.csproj
@@ -6,7 +6,7 @@
 
     <LangVersion>latestmajor</LangVersion>
 
-    <TargetFrameworks>net10.0;net8.0;net9.0</TargetFrameworks>
+    <TargetFrameworks>net48;net10.0;net8.0;net9.0</TargetFrameworks>
   </PropertyGroup>
 
   <ItemGroup>
@@ -18,6 +18,11 @@
     </PackageReference>
   </ItemGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)' == 'net48'">
+    <PackageReference Include="System.Net.Http" Version="4.3.4" />
+    <PackageReference Include="System.Text.Json" Version="10.0.2" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\Personnummer\Personnummer.csproj" />
   </ItemGroup>
diff --git a/Personnummer.Tests/PersonnummerTests.cs b/Personnummer.Tests/PersonnummerTests.cs
index de5bde3..761a6ac 100644
--- a/Personnummer.Tests/PersonnummerTests.cs
+++ b/Personnummer.Tests/PersonnummerTests.cs
@@ -169,7 +169,7 @@ public void TestMaleFemale(PersonnummerData ssn)
         [ClassData(typeof(ValidSsnDataProvider))]
         public void TestSeparator(PersonnummerData ssn)
         {
-            string sep = ssn.SeparatedFormat.Contains('+') ? "+" : "-";
+            string sep = ssn.SeparatedFormat.Contains("+") ? "+" : "-";
             Assert.Equal(sep, Personnummer.Parse(ssn.LongFormat, new Personnummer.Options { AllowCoordinationNumber = true }).Separator);
             Assert.Equal(sep, Personnummer.Parse(ssn.SeparatedLong, new Personnummer.Options { AllowCoordinationNumber = true }).Separator);
             Assert.Equal(sep, Personnummer.Parse(ssn.SeparatedFormat, new Personnummer.Options { AllowCoordinationNumber = true }).Separator);
diff --git a/Personnummer.Tests/TestTimeProvider.cs b/Personnummer.Tests/TestTimeProvider.cs
index e003612..8e70ea5 100644
--- a/Personnummer.Tests/TestTimeProvider.cs
+++ b/Personnummer.Tests/TestTimeProvider.cs
@@ -14,8 +14,8 @@ namespace Personnummer.Tests;
 public class TestTimeProvider : TimeProvider
 {
     internal DateTimeOffset Now { get; set; } = new(
-        new DateOnly(2025, 1, 1),
-        new TimeOnly(0, 0, 0, 1),
+        2025, 1, 1,
+        0, 0, 0, 1,
         TimeSpan.Zero
     );