using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var billingYear = 2023;

        var billingMonth = 1;

        var billingPeriodStart = new DateTime(billingYear, billingMonth, 1);

        var billingPeriodEnd = billingPeriodStart.AddMonths(1);

​

        //var endpointObject = new

        //{

        //  // 11 of 31 days active

        //  ActiveFrom = new DateTime(2018, 04, 27),

        //  ActiveTo = (DateTime?)new DateTime(2023, 01, 11)

        //};

        //var endpointObject = new

        //{

        //  // 5 of 31 days active

        //  ActiveFrom = new DateTime(2023, 01, 27),

        //  ActiveTo = (DateTime?)null

        //};

        //var endpointObject = new

        //{

        //  // 31 of 31 days active

        //  ActiveFrom = new DateTime(2018, 04, 05),

        //  ActiveTo = (DateTime?)null

        //};

        var endpointObject = new

        {

            // 31 of 31 days active

            ActiveFrom = new DateTime(2018, 04, 05),

            ActiveTo = (DateTime?)new DateTime(2032, 12, 31)

        };

​

        if (endpointObject.ActiveFrom > billingPeriodEnd)

        {

            return;

        }

        if (endpointObject.ActiveTo < billingPeriodStart)

        {

            return;

        }

        var activePeriodStart = billingPeriodStart;

        if (endpointObject.ActiveFrom > billingPeriodStart)

        {

            activePeriodStart = endpointObject.ActiveFrom;

        }

​

        var activePeriodEnd = billingPeriodEnd;

        if (endpointObject.ActiveTo < activePeriodEnd)

        {

            activePeriodEnd = endpointObject.ActiveTo.Value.AddDays(1); // Fraction of month, need to include the last day too.

        }

​

        var daysActive = (activePeriodEnd - activePeriodStart).TotalDays;

        var daysInMonth = DateTime.DaysInMonth(billingYear, billingMonth);

        var fractionOfMonth = (decimal)daysActive / daysInMonth;

        Console.WriteLine($"Active from: {activePeriodStart}");

        Console.WriteLine($"Active to: {activePeriodEnd}");

        Console.WriteLine($"{daysActive} of {daysInMonth} days active = {fractionOfMonth:0.0000} = {(fractionOfMonth * 100m):0.00}%");

    }

}