geissingert

public enum DateInterval
{
    Day,
    DayOfYear,
    Hour,
    Minute,
    Month,
    Quarter,
    Second,
    Weekday,
    WeekOfYear,
    Year,
}
 
private DateTime DateAdd(DateInterval interval, double number, DateTime dateValue)
{
    DateTime dtm = dateValue;
    // DateInterval.DayOfYear and DateInterval.Weekday appear the same
    // as DateInterval.Day.
    // Looking at http://msdn.microsoft.com/en-us/library/hcxe65wz(v=vs.71).aspx 
    // also seems to indicate they are the same as DateInterval.Day.
    switch (interval)
    {
        case DateInterval.Day:
        case DateInterval.DayOfYear:
        case DateInterval.Weekday:
            dtm = dateValue.AddDays(number);
            break;
        case DateInterval.Hour:
            dtm = dateValue.AddHours(number);
            break;
        case DateInterval.Minute:
            dtm = dateValue.AddMinutes(number);
            break;
        case DateInterval.Month:
            int months = Convert.ToInt32(number);
            dtm = dateValue.AddMonths(months);
            break;
        case DateInterval.Quarter:
            int quarters = Convert.ToInt32(number);
            dtm = dateValue.AddMonths(quarters * 3);
            break;
        case DateInterval.Second:
            dtm = dateValue.AddSeconds(number);
            break;
        case DateInterval.WeekOfYear:
            int weekOfYear = Convert.ToInt32(number);
            dtm = dateValue.AddDays(weekOfYear * 7);
            break;
        case DateInterval.Year:
            int years = Convert.ToInt32(number);
            dtm = dateValue.AddYears(years);
            break;
    }
    return dtm;
}
 
private DateTime DateAdd(string interval, double number, DateTime dateValue)
{
    Dictionary<string, DateInterval> dct = new Dictionary<string, DateInterval>
    {
        {"d", DateInterval.Day},
        {"y", DateInterval.DayOfYear},
        {"h", DateInterval.Hour},
        {"n", DateInterval.Minute},
        {"m", DateInterval.Month},
        {"q", DateInterval.Quarter},
        {"s", DateInterval.Second},
        {"w", DateInterval.Weekday},
        {"ww", DateInterval.WeekOfYear},
        {"yyyy", DateInterval.Year},
    };
    DateInterval di = DateInterval.Day;
    if (dct.ContainsKey(interval))
    {
        di = dct[interval];
    }
    else
    {
        throw new ArgumentException("Argument 'interval' is not a valid value.");
    }
    return DateAdd(di, number, dateValue);
}