xUnit tests for calc business days https://stackoverflow.com/a/1646396/1498252

UnitTestCalcBusinessDays.cs

namespace TestGetBusinessDays
{
    public class UnitTestCalcBusinessDays
    {
        int GetBusinessDays(DateTime startD, DateTime endD)
        {
            double calcBusinessDays =
                1 + ((endD - startD).TotalDays * 5 -
                (startD.DayOfWeek - endD.DayOfWeek) * 2) / 7;

            if (endD.DayOfWeek == DayOfWeek.Saturday) calcBusinessDays--;
            if (startD.DayOfWeek == DayOfWeek.Sunday) calcBusinessDays--;

            return Convert.ToInt32(calcBusinessDays);
        }

        int GetBusinessDays(DateOnly startD, DateOnly endD)
        {
            double calcBusinessDays =
                1 + ((endD.DayNumber - startD.DayNumber) * 5 -
                (startD.DayOfWeek - endD.DayOfWeek) * 2) / 7;

            if (endD.DayOfWeek == DayOfWeek.Saturday) calcBusinessDays--;
            if (startD.DayOfWeek == DayOfWeek.Sunday) calcBusinessDays--;

            return Convert.ToInt32(calcBusinessDays);
        }

        public static TheoryData<DateTime, DateTime, int> DateTimeTheory = new()
        {
            { new DateTime(2024,1,1), new DateTime(2024,1,1), 1 }, // same day (Monday - Monday)
            { new DateTime(2024,1,1), new DateTime(2024,1,8), 6 }, // Monday - Monday
            { new DateTime(2024,1,1), new DateTime(2024,1,7), 5 }, // Monday - Sunday
            { new DateTime(2024,1,5), new DateTime(2024,1,8), 2 }, // Friday - Monday
            { new DateTime(2024,1,8), new DateTime(2024,1,12), 5 }, // Monday - Friday
            { new DateTime(2024,2,26), new DateTime(2024,3,1), 5 }, // Monday - Friday (February has 29 days)
            { new DateTime(2024,1,7), new DateTime(2024,1,8), 1 }, // Sunday - Monday
            { new DateTime(2024,1,6), new DateTime(2024,1,8), 1 }, // Saturday - Monday
            { new DateTime(2024,1,26), new DateTime(2024,2,1), 5 }, // Friday - Thursday
            { new DateTime(2024,1,24), new DateTime(2024,1,30), 5 }, // Wednesday - Tuesday
            { new DateTime(2024,1,24), new DateTime(2024,1,31), 6 }, // Wednesday - Wednesday
        };

        [Theory, MemberData(nameof(DateTimeTheory))]
        public void TestDateTime(DateTime startD, DateTime endD, int expected)
        {
            Assert.Equal(expected, GetBusinessDays(startD, endD));
        }

        [Theory, MemberData(nameof(DateTimeTheory))]
        public void TestDateOnly(DateTime startD, DateTime endD, int expected)
        {
            Assert.Equal(expected, GetBusinessDays(DateOnly.FromDateTime(startD), DateOnly.FromDateTime(endD)));
        }

    }
}