for loop ???

Question

Hallo can someone please help me, I've got this table:

Valuation Date    Construction
30/03/2009    30,625.26    
30/04/2009    269,825.04    
30/05/2009    850,056.45    
30/06/2009    1,922,475.06    
30/07/2009    3,500,852.19    
30/08/2009    5,757,055.16    
30/09/2009    8,699,041.1    
30/10/2009    12,232,330.17    
30/11/2009    16,612,763.46    
30/12/2009    19,166,542.19    
30/01/2010    22,098,191.54    
28/02/2010    33,539,289.47    
30/03/2010    40,518,025    
30/04/2010    48,372,377.98    
30/05/2010    56,543,248.62    
30/06/2010    65,507,314.28    
30/07/2010    74,607,415.4    
30/08/2010    84,352,063.34    
30/09/2010    94,325,187.87    
30/10/2010    104,060,459.19    
30/11/2010    114,040,884.2    
30/12/2010    119,094,482.8    
30/01/2011    124,342,210.61    
28/02/2011    140,316,539.09    
30/03/2011    146,909,564.95    

Calculated from this formula:

<code>
public string Calculation(double b, DateTime validation, DateTime start, DateTime end, double alpha, double beta)
    {
            //start date(t1)
        System.TimeSpan sp_start =
            validation.Subtract(start);
        string sp_start_str = sp_start.TotalDays.ToString("#");
        double t1 = double.Parse(sp_start_str);

            // change accordingly
        if (validation.Month == 12)
            t1 = t1 - 14;
        if (validation.Month == 01)
            t1 = t1 - 28;

            // end date(t2)
        System.TimeSpan sp_end =
            end.Subtract(start);
        string sp_end_str = sp_end.TotalDays.ToString("#");
        double t2 = double.Parse(sp_end_str);
        t2 = t2 - 28;

        double divide = (t1) / (t2);
        double minus1 = (1 - (divide));
        double pow1 = 1 - (Math.Pow(minus1, alpha));
        double calculation = Math.Round(b * (Math.Pow(pow1, beta)), 2);
        
        return String.Format("{0:#,###,###,###.##}", calculation);
    }
</code>

What i need to do is:
The first January, all the months following t2 = 28;
After the second January, all the months following t2 = 56;

I probably need to use a for loop??? But not sure how to go about this.
Please someone help!!

Thanks.