diff --git a/src/Numerous.Tests/TimeUtilTests.cs b/src/Numerous.Tests/TimeUtilTests.cs
new file mode 100644
index 0000000..f84e518
--- /dev/null
+++ b/src/Numerous.Tests/TimeUtilTests.cs
@@ -0,0 +1,122 @@
+// Copyright (C) Pasi4K5 <https://www.github.com/Pasi4K5>
+// This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
+// This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
+// You should have received a copy of the GNU General Public License along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+using Numerous.Util;
+
+namespace Numerous.Tests;
+
+public sealed class TimeUtilTests
+{
+    private static readonly (
+        DateTime now,
+        ushort? year,
+        byte? month,
+        byte? day,
+        byte? hour,
+        byte? minute,
+        byte? second,
+        DateTime expected
+        )[] _parametersToDateTimeTestCases =
+        [
+            (
+                new(2000, 2, 2, 12, 10, 20),
+                2010, null, null, null, null, null,
+                new(2010, 1, 1, 0, 0, 0)
+            ),
+            (
+                new(2000, 2, 2, 12, 10, 20),
+                null, null, null, null, null, 30,
+                new(2000, 2, 2, 12, 10, 30)
+            ),
+            (
+                new(2000, 2, 2, 0, 10, 20),
+                null, null, null, 12, null, null,
+                new(2000, 2, 2, 12, 0, 0)
+            ),
+            (
+                new(2000, 2, 2, 0, 10, 20),
+                null, null, null, 12, 20, null,
+                new(2000, 2, 2, 12, 20, 0)
+            ),
+            (
+                new(2000, 2, 2, 12, 10, 20),
+                null, null, null, 10, null, null,
+                new(2000, 2, 3, 10, 0, 0)
+            ),
+            (
+                new(2000, 2, 2, 12, 10, 20),
+                null, null, null, 10, null, null,
+                new(2000, 2, 3, 10, 0, 0)
+            ),
+            (
+                new(2000, 2, 2, 12, 10, 20),
+                null, null, null, 12, null, null,
+                new(2000, 2, 3, 12, 0, 0)
+            ),
+            (
+                new(2000, 2, 2, 12, 10, 20),
+                null, 1, null, null, null, null,
+                new(2001, 1, 1, 0, 0, 0)
+            ),
+            (
+                new(2000, 2, 2, 12, 10, 20),
+                null, null, null, 12, 5, null,
+                new(2000, 2, 3, 12, 5, 0)
+            ),
+            (
+                new(2000, 12, 31, 23, 59, 50),
+                null, null, null, null, null, 10,
+                new(2001, 1, 1, 0, 0, 10)
+            ),
+            (
+                new(2000, 2, 2, 12, 10, 20),
+                null, 2, null, null, null, null,
+                new(2001, 2, 1, 0, 0, 0)
+            ),
+        ];
+
+    public static IEnumerable<object[]> ParametersToDateData =>
+        _parametersToDateTimeTestCases.Select(x => new object[] { x.now, x.year, x.month, x.day, x.hour, x.minute, x.second, x.expected });
+
+    [Theory]
+    [MemberData(nameof(ParametersToDateData))]
+    public void ParametersToDateTime_ValidParameters_ReturnsCorrectDateTime(
+        DateTime now,
+        ushort? year,
+        byte? month,
+        byte? day,
+        byte? hour,
+        byte? minute,
+        byte? second,
+        DateTime expected
+    )
+    {
+        var result = TimeUtil.ParametersToDateTime(now, year, month, day, hour, minute, second);
+
+        result.Should().Be(expected);
+    }
+
+    [Fact]
+    public void ParamtersToDateTime_AllNullParameters_ThrowsArgumentNullException()
+    {
+        Action act = () => TimeUtil.ParametersToDateTime(
+            new(2000, 2, 2, 12, 10, 20),
+            null, null, null, null, null, null
+        );
+
+        act.Should().Throw<ArgumentNullException>();
+    }
+
+    [Fact]
+    public void ParamtersToDateTime_PastDate_ThrowsArgumentOutOfRangeException()
+    {
+        Action act = () => TimeUtil.ParametersToDateTime(
+            new(2000, 2, 2, 12, 10, 20),
+            1999, null, null, null, null, null
+        );
+
+        act.Should().Throw<ArgumentOutOfRangeException>();
+    }
+}