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public static byte[] GetMonthlyFinancialReport(
        //...
        DateTime dateInterested)
    {
// ...

        DateTime monthAgoDate = dateInterested;

        int dayInterested = dateInterested.Day;
        int daysInMonthInterested = DateTime.DaysInMonth(dateInterested.Year, dateInterested.Month);
        int daysInPreviousMonth = DateTime.DaysInMonth(dateInterested.Year, (dateInterested.Month == 1) ? 12 : (dateInterested.Month - 1));

        if (dayInterested == daysInMonthInterested)
        {
            monthAgoDate = monthAgoDate.AddDays(-1 * monthAgoDate.Day);
        }
        else
        {
            monthAgoDate = monthAgoDate.AddDays(-1 * Math.Max(daysInPreviousMonth, Math.Min(daysInMonthInterested, dayInterested)));
        }

// ...
    }