DateDiff

/// <summary>
/// DateDiff in SQL style. 
/// Datepart implemented: 
///     "year" (abbr. "yy", "yyyy"), 
///     "quarter" (abbr. "qq", "q"), 
///     "month" (abbr. "mm", "m"), 
///     "day" (abbr. "dd", "d"), 
///     "week" (abbr. "wk", "ww"), 
///     "hour" (abbr. "hh"), 
///     "minute" (abbr. "mi", "n"), 
///     "second" (abbr. "ss", "s"), 
///     "millisecond" (abbr. "ms").
/// </summary>
/// <param name="DatePart"></param>
/// <param name="EndDate"></param>
/// <returns></returns>
public static Int64 DateDiff(this DateTime StartDate, String DatePart, DateTime EndDate)
{
    Int64 DateDiffVal = 0;
    System.Globalization.Calendar cal = System.Threading.Thread.CurrentThread.CurrentCulture.Calendar;
    TimeSpan ts = new TimeSpan(EndDate.Ticks - StartDate.Ticks);
    switch (DatePart.ToLower().Trim())
    {
        #region year
        case "year":
        case "yy":
        case "yyyy":
            DateDiffVal = (Int64)(cal.GetYear(EndDate) - cal.GetYear(StartDate));
            break;
        #endregion

        #region quarter
        case "quarter":
        case "qq":
        case "q":
            DateDiffVal = (Int64)((((cal.GetYear(EndDate)
                                - cal.GetYear(StartDate)) * 4)
                                + ((cal.GetMonth(EndDate) - 1) / 3))
                                - ((cal.GetMonth(StartDate) - 1) / 3));
            break;
        #endregion

        #region month
        case "month":
        case "mm":
        case "m":
            DateDiffVal = (Int64)(((cal.GetYear(EndDate)
                                - cal.GetYear(StartDate)) * 12
                                + cal.GetMonth(EndDate))
                                - cal.GetMonth(StartDate));
            break;
        #endregion

        #region day
        case "day":
        case "d":
        case "dd":
            DateDiffVal = (Int64)ts.TotalDays;
            break;
        #endregion

        #region week
        case "week":
        case "wk":
        case "ww":
            DateDiffVal = (Int64)(ts.TotalDays / 7);
            break;
        #endregion

        #region hour
        case "hour":
        case "hh":
            DateDiffVal = (Int64)ts.TotalHours;
            break;
        #endregion

        #region minute
        case "minute":
        case "mi":
        case "n":
            DateDiffVal = (Int64)ts.TotalMinutes;
            break;
        #endregion

        #region second
        case "second":
        case "ss":
        case "s":
            DateDiffVal = (Int64)ts.TotalSeconds;
            break;
        #endregion

        #region millisecond
        case "millisecond":
        case "ms":
            DateDiffVal = (Int64)ts.TotalMilliseconds;
            break;
        #endregion

        default:
            throw new Exception(String.Format("DatePart \"{0}\" is unknown", DatePart));
    }
    return DateDiffVal;
}

// Gets the total days from 01/01/2000.
DateTime dt = new DateTime(2000, 01, 01);
Int64 days = dt.DateDiff("day", DateTime.Now);
// Gets the total hours from 01/01/2000.
Int64 hours = dt.DateDiff("hour", DateTime.Now);