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.