Days 360 Excel

AbstractDays360.cs

internal abstract class AbstractDays360
{
    public abstract int Calculate(DateTime start, DateTime end);

    protected static int Calculate(DateTime start, DateTime end, int startDay, int endDay)
    {
        return (end.Year - start.Year) * 360 + (end.Month - start.Month) * 30 + (endDay - startDay);
    }

    protected static bool LastDayOfFebruary(DateTime date)
    {
        return date.Equals(new DateTime(date.Year,3, 1).AddDays(-1));
    }
}

USExcelCompatible.cs

using System;

internal class USExcelCompatible : AbstractDays360
{
    public override int Calculate(DateTime start, DateTime end)
    {
        var startDay = start.Day;
        var endDay = end.Day;

        if (startDay == 31 || LastDayOfFebruary(start))
            startDay = 30;

        if (startDay == 30 && endDay == 31)
            endDay = 30;

        return Calculate(start, end, startDay, endDay);
    }
}