You will not get the Last Date like this. First you need to get the Last day in a month and then you need to concatenate the Day with search parameters.
Please refer this code
int year = 2014;
int month = 8;
int day = DateTime.DaysInMonth(year, month);
DateTime date = Convert.ToDateTime(month + "/" + day + "/" + year);
string datewithDDMMYYY = date.ToString("dd/MM/yyyy",new CultureInfo("en-GB"));