using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2025, 7, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2024, 7, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2024, 6, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2024, 5, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2024, 4, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2024, 3, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2024, 2, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2024, 1, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 12, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 11, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 10, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 9, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 8, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 7, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 6, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 5, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 4, 5)));

        Console.WriteLine("-------------------------");     

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 3, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 2, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2023, 1, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 12, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 11, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 10, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 9, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 8, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 7, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 6, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 5, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 4, 5)));

        Console.WriteLine("-------------------------");

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 3, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 2, 5)));

        Console.WriteLine(BasePeriod.Determine(new DateTime(2024, 7, 5), new DateTime(2022, 1, 5)));

    }

}

​

public enum BasePeriodType

{

    Unknown,

    Regular,

    Alternate,

    Extended

}

​

public static class BasePeriod

{

    public static BasePeriodType Determine(DateTime effectiveDate, DateTime startOfBasePeriod)

    {

        int effectiveDateQuarter = (int) Math.Ceiling(effectiveDate.Month / 3m);

        int startOfBasePeriodQuarter = (int) Math.Ceiling(startOfBasePeriod.Month / 3m);

        var yearDifference = effectiveDate.Year - startOfBasePeriod.Year;

        var numberOfQuartersAgo = ((4 - startOfBasePeriodQuarter) + effectiveDateQuarter) + (4 * (yearDifference - 1));

        if(numberOfQuartersAgo == 4){

            return BasePeriodType.Alternate;

        }

        if(numberOfQuartersAgo == 5) {

            return BasePeriodType.Regular;

        }

        if(numberOfQuartersAgo > 5) {

            return BasePeriodType.Extended;

        }

        return BasePeriodType.Unknown;

    }

}