DateTime.Truncate

    public static DateTime Truncate(this DateTime date, DateTimeResolution resolution = DateTimeResolution.Hour)
    {
      switch (resolution)
      {
        case DateTimeResolution.Year:
          return new DateTime(date.Year, 1, 1);

        case DateTimeResolution.Month:
          return new DateTime(date.Year, date.Month, 1);

        case DateTimeResolution.Day:
          return date.Date;

        case DateTimeResolution.Hour:
          return new DateTime(date.Ticks - date.Ticks % TimeSpan.TicksPerHour);

        case DateTimeResolution.Minute:
          return new DateTime(date.Ticks - date.Ticks % TimeSpan.TicksPerMinute);

        case DateTimeResolution.Second:
          return new DateTime(date.Ticks - date.Ticks % TimeSpan.TicksPerSecond);

        case DateTimeResolution.Millisecond:
          return new DateTime(date.Ticks - date.Ticks % TimeSpan.TicksPerMillisecond);

        default:
          throw new ArgumentOutOfRangeException("resolution", resolution, null);
      }
    }
    
    public enum DateTimeResolution
    {
      Year,
      Month,
      Day,
      Hour,
      Minute,
      Second,
      Millisecond
    }