public static void Main()
var testDate = DateTime.Parse("1/11/2021");
Console.Write(getProcessingDate(testDate).ToString());
private static DateTime getProcessingDate(DateTime dateTimeProvider)
int processingDay = dateTimeProvider.Day;
return new DateTime(dateTimeProvider.Year, dateTimeProvider.Month, 1);
else if (processingDay < 21)
return new DateTime(dateTimeProvider.Year, dateTimeProvider.Month, 11);
return new DateTime(dateTimeProvider.Year, dateTimeProvider.Month, 21);