AppsheetEpplus/FormulaParsing/Excel/Functions/DateTime/Days360.cs

using System.Collections.Generic;
using System.Globalization;
using System.Linq;

namespace AppsheetEpplus;

public class Days360 : ExcelFunction {
  private enum Days360Calctype {
    European,
    Us,
  }

  public override CompileResult Execute(
      IEnumerable<FunctionArgument> arguments,
      ParsingContext context) {
    ValidateArguments(arguments, 2);
    var numDate1 = ArgToDecimal(arguments, 0);
    var numDate2 = ArgToDecimal(arguments, 1);
    var dt1 = System.DateTime.FromOADate(numDate1);
    var dt2 = System.DateTime.FromOADate(numDate2);

    var calcType = Days360Calctype.Us;
    if (arguments.Count() > 2) {
      var european = ArgToBool(arguments, 2);
      if (european) {
        calcType = Days360Calctype.European;
      }
    }

    var startYear = dt1.Year;
    var startMonth = dt1.Month;
    var startDay = dt1.Day;
    var endYear = dt2.Year;
    var endMonth = dt2.Month;
    var endDay = dt2.Day;

    if (calcType == Days360Calctype.European) {
      if (startDay == 31) {
        startDay = 30;
      }
      if (endDay == 31) {
        endDay = 30;
      }
    } else {
      var calendar = new GregorianCalendar();
      var nDaysInFeb = calendar.IsLeapYear(dt1.Year) ? 29 : 28;

      // If the investment is EOM and (Date1 is the last day of February) and (Date2 is the last day of February), then change D2 to 30.
      if (startMonth == 2 && startDay == nDaysInFeb && endMonth == 2 && endDay == nDaysInFeb) {
        endDay = 30;
      }
      // If the investment is EOM and (Date1 is the last day of February), then change D1 to 30.
      if (startMonth == 2 && startDay == nDaysInFeb) {
        startDay = 30;
      }
      // If D2 is 31 and D1 is 30 or 31, then change D2 to 30.
      if (endDay == 31 && (startDay == 30 || startDay == 31)) {
        endDay = 30;
      }
      // If D1 is 31, then change D1 to 30.
      if (startDay == 31) {
        startDay = 30;
      }
    }
    var result =
        (endYear * 12 * 30 + endMonth * 30 + endDay)
            - (startYear * 12 * 30 + startMonth * 30 + startDay);
    return CreateResult(result, DataType.Integer);
  }
}