DateTimeFloor;DateTimeCeiling

public static class DateExtensions
{
    static public DateTime DateTimeFloor(this DateTime dt, TimeInterval Interval)
    {
        return WorkMethod(dt, 0L, Interval);
    }
    static public DateTime DateTimeMidpoint(this DateTime dt, TimeInterval Interval)
    {
        return WorkMethod(dt, 2L, Interval);
    }
    static public DateTime DateTimeCeiling(this DateTime dt, TimeInterval Interval)
    {
        return WorkMethod(dt, 1L, Interval);
    }
    static public DateTime DateTimeCeilingUnbounded(this DateTime dt, TimeInterval Interval)
    {
        return WorkMethod(dt, 1L, Interval).AddTicks(-1);
    }
    static public DateTime DateTimeRound(this DateTime dt, TimeInterval Interval)
    {
        if (dt >= WorkMethod(dt, 2L, Interval))
            return WorkMethod(dt, 1L, Interval);
        else
            return WorkMethod(dt, 0L, Interval);
    }
    public enum TimeInterval : long
    {
        YearFromJanuary = 120L,
        YearFromFebruary = 121L,
        YearFromMarch = 122L,
        YearFromApril = 123L,
        YearFromMay = 124L,
        YearFromJune = 125L,
        YearFromJuly = 126L,
        YearFromAugust = 127L,
        YearFromSeptember = 128L,
        YearFromOctober = 129L,
        YearFromNovember = 130L,
        YearFromDecember = 131L,
        HalfYearFromJanuary = 60L,
        HalfYearFromFebruary = 61L,
        HalfYearFromMarch = 62L,
        HalfYearFromApril = 63L,
        HalfYearFromMay = 64L,
        HalfYearFromJune = 65L,
        QuarterYearFromJanuary = 30L,
        QuarterYearFromFebruary = 31L,
        QuarterYearFromMarch = 32L,
        BiMonthlyFromJanuary = 20L,
        BiMonthlyFromFebruary = 21L,
        OneMonth = 10L,
        OneWeekFromSunday = 1L,
        OneWeekFromMonday = 2L,
        OneWeekFromTuesday = 3L,
        OneWeekFromWednesday = 4L,
        OneWeekFromThursday = 5L,
        OneWeekFromFriday = 6L,
        OneWeekFromSaturday = 7L,
        OneDay = TimeSpan.TicksPerDay,
        TwelveHour = TimeSpan.TicksPerHour * 12L,
        SixHour = TimeSpan.TicksPerHour * 6L,
        ThreeHour = TimeSpan.TicksPerHour * 3L,
        OneHour = TimeSpan.TicksPerHour,
        HalfHour = TimeSpan.TicksPerMinute * 30L,
        QuarterHour = TimeSpan.TicksPerMinute * 15L,
        OneMinute = TimeSpan.TicksPerMinute,
        HalfMinute = TimeSpan.TicksPerSecond * 30L,
        QuarterMinute = TimeSpan.TicksPerSecond * 15L,
        OneSecond = TimeSpan.TicksPerSecond,
        TenthOfASecond = TimeSpan.TicksPerSecond / 10L,
        HundrethOfASecond = TimeSpan.TicksPerSecond / 100L,
        ThousandthOfASecond = TimeSpan.TicksPerSecond / 1000L
    }
    static private DateTime WorkMethod(DateTime dt, long ReturnType, TimeInterval Interval)
    {
        long Interval1 = (long)Interval;
        long TicksFromFloor = 0L;
        int IntervalFloor = 0;
        int FloorOffset = 0;
        int IntervalLength = 0;
        DateTime floorDate;
        DateTime ceilingDate;

        if (Interval1 > 132L) //Set variables to calculate date for time interval less than one day.
        {
            floorDate = new DateTime(dt.Ticks - (dt.Ticks % Interval1), dt.Kind);
            if (ReturnType != 0L)
                TicksFromFloor = Interval1 / ReturnType;
        }
        else if (Interval1 < 8L) //Set variables to calculate date for time interval of one week.
        {
            IntervalFloor = (int)(Interval1) - 1;
            FloorOffset = (int)dt.DayOfWeek * -1;
            floorDate = new DateTime(dt.Year, dt.Month, dt.Day, 0, 0, 0, dt.Kind).AddDays(-(IntervalFloor > FloorOffset ? FloorOffset + 7 - IntervalFloor : FloorOffset - IntervalFloor));
            if (ReturnType != 0L)
                TicksFromFloor = TimeSpan.TicksPerDay * 7L / ReturnType;
        }
        else //Set variables to calculate date for time interval one month or greater.
        {
            IntervalLength = Interval1 >= 130L ? 12 : (int)(Interval1 / 10L);
            IntervalFloor = (int)(Interval1 % IntervalLength);
            FloorOffset = (dt.Month - 1) % IntervalLength;
            floorDate = new DateTime(dt.Year, dt.Month, 1, 0, 0, 0, dt.Kind).AddMonths(-(IntervalFloor > FloorOffset ? FloorOffset + IntervalLength - IntervalFloor : FloorOffset - IntervalFloor));
            if (ReturnType != 0L)
            {
                ceilingDate = floorDate.AddMonths(IntervalLength);
                TicksFromFloor = (long)ceilingDate.Subtract(floorDate).Ticks / ReturnType;
            }
        }
        return floorDate.AddTicks(TicksFromFloor);
    }
}

DateTime currentHour = DateTime.Now.DateTimeFloor(DateExtensions.TimeInterval.OneHour); // Returns the current date and time with the minutes and seconds set to zero.
DateTime nextYear = DateTime.Now.DateTimeCeiling(DateExtensions.TimeInterval.YearFromJanuary); // Returns January 1 of next year at midnight.