Useful date and time helper functions in C#.

using System;

public enum Quarter {
    First = 1,
    Second = 2,
    Third = 3,
    Fourth = 4
}

public enum Month {
    January = 1,
    February = 2,
    March = 3,
    April = 4,
    May = 5,
    June = 6,
    July = 7,
    August = 8,
    September = 9,
    October = 10,
    November = 11,
    December = 12
}

public class DateHelper {
    public static readonly DateTime SqlMinDate = new DateTime(1753, 1, 1);
    public static readonly DateTime SqlMaxDate = new DateTime(1999, 12, 31);

    #region Quarters

    public static DateTime GetStartOfQuarter(int Year, Quarter Qtr) {
        switch (Qtr) {
            case Quarter.First:
                return new DateTime(Year, 1, 1, 0, 0, 0, 0);
            case Quarter.Second:
                return new DateTime(Year, 4, 1, 0, 0, 0, 0);
            case Quarter.Third:
                return new DateTime(Year, 7, 1, 0, 0, 0, 0);
            default:
                return new DateTime(Year, 10, 1, 0, 0, 0, 0);
        }
    }

    public static DateTime GetEndOfQuarter(int Year, Quarter Qtr) {
        switch (Qtr) {
            case Quarter.First:
                return new DateTime(Year, 3, DateTime.DaysInMonth(Year, 3), 23, 59, 59, 999);
            case Quarter.Second:
                return new DateTime(Year, 6, DateTime.DaysInMonth(Year, 6), 23, 59, 59, 999);
            case Quarter.Third:
                return new DateTime(Year, 9, DateTime.DaysInMonth(Year, 9), 23, 59, 59, 999);
            default:
                return new DateTime(Year, 12, DateTime.DaysInMonth(Year, 12), 23, 59, 59, 999);
        }
    }

    public static Quarter GetQuarter(Month Month) {
        if (Month <= Month.March) {
            // 1st Quarter = January 1 to March 31
            return Quarter.First;
        }

        if ((Month >= Month.April) && (Month <= Month.June)) {
            // 2nd Quarter = April 1 to June 30
            return Quarter.Second;
        }
        if ((Month >= Month.July) && (Month <= Month.September)) {
            // 3rd Quarter = July 1 to September 30
            return Quarter.Third;
        }

        return Quarter.Fourth;
    }

    public static DateTime GetEndOfLastQuarter() {
        return (Month) DateTime.Now.Month <= Month.March ? GetEndOfQuarter(DateTime.Now.Year - 1, Quarter.Fourth) : GetEndOfQuarter(DateTime.Now.Year, GetQuarter((Month) DateTime.Now.Month));
    }

    public static DateTime GetStartOfLastQuarter() {
        return (Month) DateTime.Now.Month <= Month.March ? GetStartOfQuarter(DateTime.Now.Year - 1, Quarter.Fourth) : GetStartOfQuarter(DateTime.Now.Year, GetQuarter((Month) DateTime.Now.Month));
    }

    public static DateTime GetStartOfCurrentQuarter() {
        return GetStartOfQuarter(DateTime.Now.Year, GetQuarter((Month) DateTime.Now.Month));
    }

    public static DateTime GetEndOfCurrentQuarter() {
        return GetEndOfQuarter(DateTime.Now.Year, GetQuarter((Month) DateTime.Now.Month));
    }

    #endregion

    #region Weeks

    public static DateTime GetStartOfLastWeek() {
        int DaysToSubtract = (int) DateTime.Now.DayOfWeek + 7;
        DateTime dt = DateTime.Now.Subtract(System.TimeSpan.FromDays(DaysToSubtract));
        return new DateTime(dt.Year, dt.Month, dt.Day, 0, 0, 0, 0);
    }

    public static DateTime GetEndOfLastWeek() {
        DateTime dt = GetStartOfLastWeek().AddDays(6);
        return new DateTime(dt.Year, dt.Month, dt.Day, 23, 59, 59, 999);
    }

    public static DateTime GetStartOfCurrentWeek() {
        int DaysToSubtract = (int) DateTime.Now.DayOfWeek;
        DateTime dt = DateTime.Now.Subtract(System.TimeSpan.FromDays(DaysToSubtract));
        return new DateTime(dt.Year, dt.Month, dt.Day, 0, 0, 0, 0);
    }

    public static DateTime GetEndOfCurrentWeek() {
        DateTime dt = GetStartOfCurrentWeek().AddDays(6);
        return new DateTime(dt.Year, dt.Month, dt.Day, 23, 59, 59, 999);
    }

    #endregion

    #region Months

    public static DateTime GetStartOfMonth(Month Month, int Year) {
        return GetStartOfMonth(Year, (int) Month);
    }

    public static DateTime GetStartOfMonth(int Month, int Year) {
        return new DateTime(Year, Month, 1, 0, 0, 0, 0);
    }

    public static DateTime GetEndOfMonth(Month Month, int Year) {
        return GetEndOfMonth(Year, (int) Month);
    }

    public static DateTime GetEndOfMonth(int Month, int Year) {
        return new DateTime(Year, Month, DateTime.DaysInMonth(Year, Month), 23, 59, 59, 999);
    }

    public static DateTime GetStartOfLastMonth() {
        return DateTime.Now.Month == 1 ? GetStartOfMonth(Month.December, DateTime.Now.Year - 1) : GetStartOfMonth(DateTime.Now.Month - 1, DateTime.Now.Year);
    }

    public static DateTime GetEndOfLastMonth() {
        return DateTime.Now.Month == 1 ? GetEndOfMonth(12, DateTime.Now.Year - 1) : GetEndOfMonth(DateTime.Now.Month - 1, DateTime.Now.Year);
    }

    public static DateTime GetStartOfCurrentMonth() {
        return GetStartOfMonth(DateTime.Now.Month, DateTime.Now.Year);
    }

    public static DateTime GetEndOfCurrentMonth() {
        return GetEndOfMonth(DateTime.Now.Month, DateTime.Now.Year);
    }

    #endregion

    #region Years

    public static DateTime GetStartOfYear(int Year) {
        return new DateTime(Year, 1, 1, 0, 0, 0, 0);
    }

    public static DateTime GetEndOfYear(int Year) {
        return new DateTime(Year, 12, DateTime.DaysInMonth(Year, 12), 23, 59, 59, 999);
    }

    public static DateTime GetStartOfLastYear() {
        return GetStartOfYear(DateTime.Now.Year - 1);
    }

    public static DateTime GetEndOfLastYear() {
        return GetEndOfYear(DateTime.Now.Year - 1);
    }

    public static DateTime GetStartOfCurrentYear() {
        return GetStartOfYear(DateTime.Now.Year);
    }

    public static DateTime GetEndOfCurrentYear() {
        return GetEndOfYear(DateTime.Now.Year);
    }

    #endregion

    #region Days

    public static DateTime GetStartOfDay(DateTime date) {
        return new DateTime(date.Year, date.Month, date.Day, 0, 0, 0, 0);
    }

    public static DateTime GetEndOfDay(DateTime date) {
        return new DateTime(date.Year, date.Month, date.Day, 23, 59, 59, 999);
    }

    #endregion
}