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

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

namespace AppsheetEpplus;

public abstract class DateParsingFunction : ExcelFunction {
  protected System.DateTime ParseDate(IEnumerable<FunctionArgument> arguments, object dateObj) {
    System.DateTime date;
    if (dateObj is string) {
      date = System.DateTime.Parse(dateObj.ToString(), CultureInfo.InvariantCulture);
    } else {
      var d = ArgToDecimal(arguments, 0);
      date = System.DateTime.FromOADate(d);
    }
    return date;
  }
}