public static class MonthExtensions
{
    /// 

    /// Gets the first day of the month.
    /// 
    /// The given date.
    /// the first day of the month
    public static DateTime GetFirstDayOfMonth(DateTime givenDate)
    {
        return new DateTime(givenDate.Year, givenDate.Month, 1);
    }

    /// 

    /// Gets the last day of month.
    /// 
    /// The given date.
    /// the last day of the month
    public static DateTime GetTheLastDayOfMonth(DateTime givenDate)
    {
        return GetFirstDayOfMonth(givenDate).AddMonths(1).Subtract(new TimeSpan(1, 0, 0, 0, 0));
    }
}