Extension Methods in C#

/// <summary>
/// Date and Time Manager class.
/// </summary>
public static class DateMan
{
  /// <summary>
  /// Returns the age in years for the given DOB
  /// </summary>
  public static int Age(this DateTime dob)
  {
    return (int)((DateTime.Now - dob).Days / 365.25);
  }
  /// <summary>
  /// Returns true if given date time falls in the morning (AM).
  /// </summary>
  public static bool IsMorning(this DateTime time)
  {
    return time.TimeOfDay < new DateTime(2000, 1, 1, 12, 0, 0).TimeOfDay;
  }
  /// <summary>
  /// Returns true if given date time falls in the afternoon (PM).
  /// </summary>
  public static bool IsAfternoon(this DateTime time)
  {
    return time.TimeOfDay >= new DateTime(2000, 1, 1, 12, 0, 0).TimeOfDay;
  }
  /// <summary>
  /// Returns the start of the next hour following the given time.
  /// </summary>
  public static DateTime StartOfNextHour(this DateTime time)
  {
    DateTime result = time.AddHours(1).AddMinutes(-time.Minute).AddSeconds(-time.Second);
    return result;
  }
  /// <summary>
  /// Returns the start of the day following the given time.
  /// </summary>
  public static DateTime StartOfNextDay(this DateTime time)
  {
    DateTime result = time.AddDays(1);
    return result.Date;
  }
  /// <summary>
  /// Returns the start of the current week.
  /// </summary>
  public static DateTime StartOfThisWeek(this DateTime time)
  {
    DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
    // Calculate difference between specified date days of week and first day of week
    int Days = time.DayOfWeek - dfi.FirstDayOfWeek;
    // Add day difference to number of days in week then divide sum with days in week, reminder is number of days we need to subtract
    Days = (7 + Days) % 7;
    // Subtract difference of days from specified date
    return time.AddDays(-Days);
  }
  /// <summary>
  /// Returns the start of the week following the given time.
  /// </summary>
  public static DateTime StartOfNextWeek(this DateTime time)
  {
    return time.StartOfThisWeek().AddDays(7);
  }
  /// <summary>
  /// Returns the start of the month for the month following the given time.
  /// </summary>
  public static DateTime StartOfNextMonth(this DateTime time)
  {
    DateTime result = time.AddMonths(1);
    return new DateTime(result.Year, result.Month, 1);
  }
  /// <summary>
  /// Returns the start of the month for the current month.
  /// </summary>
  public static DateTime StartOfThisMonth(this DateTime time)
  {
    return new DateTime(time.Year, time.Month, 1);
  }
  /// <summary>
  /// Returns the end of the month for the current month.
  /// </summary>
  public static DateTime EndOfThisMonth(this DateTime time)
  {
    DateTime result = time.AddMonths(1);
    return new DateTime(result.Year, result.Month, 1).AddDays(-1);
  }
  /// <summary>
  /// Returns the start of the year for the year following the given time.
  /// </summary>
  public static DateTime StartOfNextYear(this DateTime time)
  {
    DateTime result = time.AddYears(1);
    return new DateTime(result.Year, 1, 1);
  }
  /// <summary>
  /// Assumes the given string is the description for a month.
  /// Returns the integer value representing that month.
  /// </summary>
  /// <remarks>
  /// A 3 letter representation is enough, whole name not needed (though accepted).
  /// </remarks>
  public static int GetMonthByDescription(this string month)
  {
    if (month.IsInteger())
    {
      return int.Parse(month);
    }
    if (month.Trim().Length <= 2)
    {
      throw new Exception("Input not a valid month representation.");
    }
    int result = 0;
    switch (month.Left(3).ToLower())
    {
      case "jan":
        result = 1;
        break;
      case "feb":
        result = 2;
        break;
      case "mar":
        result = 3;
        break;
      case "apr":
        result = 4;
        break;
      case "may":
        result = 5;
        break;
      case "jun":
        result = 6;
        break;
      case "jul":
        result = 7;
        break;
      case "aug":
        result = 8;
        break;
      case "sep":
        result = 9;
        break;
      case "oct":
        result = 10;
        break;
      case "nov":
        result = 11;
        break;
      case "dec":
        result = 12;
        break;
    }
    return result;
  }
  /// <summary>
  /// Parses a millisecond value into Days, Hours, Minutes, Seconds and Milliseconds.
  /// </summary>
  public static void MiliSecParse(int inputMiliSecs, out int days, out int hours, out int minutes, out int seconds, out int miliSeconds)
  {
    TimeSpan timeSpan = new TimeSpan(0, 0, 0, 0, inputMiliSecs);
    MiliSecParse(timeSpan, out days, out hours, out minutes, out seconds, out miliSeconds);
  }
  /// <summary>
  /// Parses a millisecond value into Days, Hours, Minutes, Seconds and Milliseconds.
  /// </summary>
  public static void MiliSecParse(double inputMiliSecs, out int days, out int hours, out int minutes, out int seconds, out int miliSeconds)
  {
    TimeSpan timeSpan = new TimeSpan(0, 0, 0, 0, (int)inputMiliSecs);
    MiliSecParse(timeSpan, out days, out hours, out minutes, out seconds, out miliSeconds);
  }
  /// <summary>
  /// Parses a millisecond value into Days, Hours, Minutes, Seconds and Milliseconds.
  /// </summary>
  public static void MiliSecParse(TimeSpan timeSpan, out int days, out int hours, out int minutes, out int seconds, out int miliSeconds)
  {
    days = timeSpan.Days;
    hours = timeSpan.Hours;
    minutes = timeSpan.Minutes;
    seconds = timeSpan.Seconds;
    miliSeconds = timeSpan.Milliseconds;
  }
  /// <summary>
  /// Returns the number of days in the month of the given date.
  /// </summary>
  public static int DaysInMonth(this DateTime date)
  {
    int result = 31;
    switch (date.Month)
    {
      case 2:
        result = date.IsLeapYear() ? 29 : 28;
        break;
      case 4:
        result = 30;
        break;
      case 6:
        result = 30;
        break;
      case 9:
        result = 30;
        break;
      case 11:
        result = 30;
        break;
    }
    return result;
  }
  /// <summary>
  /// Returns true if the given date falls in a leap year.
  /// </summary>
  public static bool IsLeapYear(this DateTime date)
  {
    int year = date.Year;
    return ((year.IsMultipleOf(4) && !year.IsMultipleOf(100)) || year.IsMultipleOf(400));
  }
  /// <summary>
  /// Returns PC time (without daylight savings)
  /// </summary>
  public static DateTime GetPCTimeNoDaylightSaving()
  {
    DateTime result = DateTime.Now;
    TimeZone timeZone = TimeZone.CurrentTimeZone;
    if (timeZone.IsDaylightSavingTime(result))
    {
      DaylightTime daylightTime = timeZone.GetDaylightChanges(result.Year);
      result -= daylightTime.Delta;
    }
    return result;
  }
  /// <summary>
  /// Returns true if the system's time zone is now experiencing a period of
  /// Daylight Savings adjustment. e.g. one hour added
  /// </summary>
  public static bool IsDaylightSavingTime()
  {
    return TimeZone.CurrentTimeZone.IsDaylightSavingTime(DateTime.Now);
  }
  /// <summary>
  /// Returns true if the system's time zone is in a period of
  /// Daylight Savings adjustment for the given date. e.g. one hour added
  /// </summary>
  public static bool IsDaylightSavingTime(DateTime date)
  {
    return TimeZone.CurrentTimeZone.IsDaylightSavingTime(date);
  }
  /// <summary>
  /// Returns the quarter of the year within which the given date falls:
  /// Jan, Feb, Mar = 1
  /// ...
  /// Oct, Nov, Dec = 4
  /// </summary>
  /// <param name="date"></param>
  /// <returns></returns>
  public static int Quarter(this DateTime date)
  {
    return (date.Month + 2) / 3;
  }
}