fix: add timeprovider fallback

Personnummer.Tests/CoordinationNumberTests.cs

@@ -122,7 +122,6 @@ public void TestParseInvalidCn(PersonnummerData ssn)
             }));
         }
 
-#if NET8_0_OR_GREATER
         [Theory]
         [ClassData(typeof(ValidCnDataProvider))]
         public void TestAgeCn(PersonnummerData ssn)
@@ -146,7 +145,6 @@ public void TestAgeCn(PersonnummerData ssn)
             // Du to age not being possible to fetch from >100 year short format without separator, we aught to check this here.
             Assert.Equal(years > 99 ? years - 100 : years, Personnummer.Parse(ssn.ShortFormat, new Personnummer.Options { AllowCoordinationNumber = true, TimeProvider = timeProvider }).Age);
         }
-#endif
 
         [Theory]
         [ClassData(typeof(ValidCnDataProvider))]

Personnummer.Tests/PersonnummerTests.cs

@@ -75,8 +75,6 @@ public void TestParseInvalid(PersonnummerData ssn)
             }));
         }
 
-
-#if NET8_0_OR_GREATER // Requires TimeProvider.
         [Theory]
         [ClassData(typeof(ValidSsnDataProvider))]
         public void TestAge(PersonnummerData ssn)
@@ -111,8 +109,6 @@ public void TestEdgeCasesAroundBirthday()
             Assert.Equal(9, new Personnummer("20800110-8516", new Personnummer.Options() {TimeProvider = timeProvider} ).Age); // Upcoming Birthday tomorrow
         }
 
-#endif
-
         [Theory]
         [ClassData(typeof(ValidSsnDataProvider))]
         public void TestFormat(PersonnummerData ssn)

Personnummer.Tests/TestTimeProvider.cs

@@ -1,8 +1,12 @@
 using System;
 
-namespace Personnummer.Tests;
-
 #if NET8_0_OR_GREATER
+using TimeProvider = System.TimeProvider;
+#else
+using TimeProvider = Personnummer.TimeProvider;
+#endif
+
+namespace Personnummer.Tests;
 
 /// <summary>
 /// TimeProvider which always returns the same date: 2025 01 01 00:00:01 with UTC timezone on local time.
@@ -16,11 +20,5 @@ public class TestTimeProvider : TimeProvider
     );
 
     public override DateTimeOffset GetUtcNow()
-    {
-        return Now;
-    }
-
-    public override TimeZoneInfo LocalTimeZone { get; } = TimeZoneInfo.Utc;
+        => Now;
 }
-
-#endif

Personnummer/Personnummer.cs

@@ -3,6 +3,12 @@
 using System.Text.RegularExpressions;
 using Personnummer.Exceptions;
 
+#if NET8_0_OR_GREATER
+using TimeProvider = System.TimeProvider;
+#else
+using TimeProvider = Personnummer.TimeProvider;
+#endif
+
 namespace Personnummer
 {
     public class Personnummer
@@ -25,20 +31,16 @@ public Options()
             /// </summary>
             public bool AllowInterimNumber { get; set; } = false;
 
-#if (NET8_0_OR_GREATER)
             /// <summary>
             /// TimeProvider to use in calculations which are time dependent.<br/>
             /// Uses `GetLocalNow` method.
             /// <remarks>
             /// Defaults to System provider.
             /// </remarks>
             /// </summary>
-            public TimeProvider TimeProvider { private get; init; } = TimeProvider.System;
+            public TimeProvider TimeProvider { private get; set; } = TimeProvider.System;
 
             internal DateTimeOffset DateTimeNow => TimeProvider.GetLocalNow();
-#else
-            internal DateTimeOffset DateTimeNow => DateTimeOffset.Now;
-#endif
         }
 
         #region Fields and Properties

Personnummer/TimeProvider.cs

@@ -0,0 +1,21 @@
+using System;
+
+namespace Personnummer;
+
+#if (!NET8_0_OR_GREATER)
+public abstract class TimeProvider
+{
+    public virtual DateTimeOffset GetLocalNow() => GetUtcNow().ToLocalTime();
+
+    public static TimeProvider System { get; }
+        = new FallbackTimeProvider();
+
+    public abstract DateTimeOffset GetUtcNow();
+
+    private sealed class FallbackTimeProvider : TimeProvider
+    {
+        public override DateTimeOffset GetUtcNow()
+            => DateTimeOffset.UtcNow;
+    }
+}
+#endif