DateTime Extensions to Make Some Simple Tasks a Little More Readable

Introduction

If you want to insert the start of today into a database, you need to pass today's date, at midnight. That isn't difficult, but the code looks clumsy, and needs to be commented:

using (SqlCommand cmd = new SqlCommand("INSERT INTO myTable VALUES (@DATE)", con))
    {
    DateTime now = DateTime.Now;
    cmd.Parameters.AddWithValue("@DATE", new DateTime(now.Year, now.Month, now.Day));
    cmd.ExecuteNonQuery();
    }

It would be much easier to read if you could just say:

using (SqlCommand cmd = new SqlCommand("INSERT INTO myTable VALUES (@DATE)", con))
    {
    cmd.Parameters.AddWithValue("@DATE", DateTime.Now.AtMidnight());
    cmd.ExecuteNonQuery();
    }

In addition, there are times when you want to know the date next Tuesday, or the first Monday of a particular month. This tip presents a small number of extension methods to do just that.

Using the Code

Add a new class file to your project, and copy the code below into it.

You can then use the extension methods as normal:

DateTime pickedDate = dateTimePicker1.Value;
Console.WriteLine("Start date             : {0}", pickedDate);
Console.WriteLine("First of month         : {0}", pickedDate.FirstOfMonth());
Console.WriteLine("First Tuesday of month : {0}", pickedDate.FirstOfMonth(DayOfWeek.Tuesday));
Console.WriteLine("Last of month          : {0}", pickedDate.LastOfMonth());
Console.WriteLine("Last Tuesday of month  : {0}", pickedDate.LastOfMonth(DayOfWeek.Tuesday));
Console.WriteLine("Next Tuesday           : {0}", pickedDate.NextDayOfWeek(DayOfWeek.Tuesday));
Console.WriteLine("Start of day           : {0}", pickedDate.AtMidnight());
Console.WriteLine("Midday                 : {0}", pickedDate.AtMidday());

The Method Code

using System;

namespace FieldFilterTest
{
/// <summary>
/// Extension methods for DateTime
/// </summary>
public static class DateTimeExtensions
    {
    #region Public Methods
    /// <summary>
    /// Returns the first day of the month
    /// </summary>
    /// <example>
    /// DateTime firstOfThisMonth = DateTime.Now.FirstOfMonth;
    /// </example>
    /// <param name="dt">Start date</param>
    /// <returns></returns>
    public static DateTime FirstOfMonth(this DateTime dt)
        {
        return (dt.AddDays(1 - dt.Day)).AtMidnight();
        }
    /// <summary>
    /// Returns the first specified day of the week in the current month
    /// </summary>
    /// <example>
    /// DateTime firstTuesday = DateTime.Now.FirstDayOfMonth(DayOfWeek.Tuesday);
    /// </example>
    /// <param name="dt">Start date</param>
    /// <param name="dayOfWeek">The required day of week</param>
    /// <returns></returns>
    public static DateTime FirstOfMonth(this DateTime dt, DayOfWeek dayOfWeek)
        {
        DateTime firstDayOfMonth = dt.FirstOfMonth();
        return (firstDayOfMonth.DayOfWeek == dayOfWeek ? firstDayOfMonth : 
                firstDayOfMonth.NextDayOfWeek(dayOfWeek)).AtMidnight();
        }
    /// <summary>
    /// Returns the last day in the current month
    /// </summary>
    /// <example>
    /// DateTime endOfMonth = DateTime.Now.LastDayOfMonth();
    /// </example>
    /// <param name="dt" />Start date
    /// <returns />
    public static DateTime LastOfMonth(this DateTime dt)
        {
        int daysInMonth = DateTime.DaysInMonth(dt.Year, dt.Month);
        return dt.FirstOfMonth().AddDays(daysInMonth - 1).AtMidnight();
        }
    /// <summary>
    /// Returns the last specified day of the week in the current month
    /// </summary>
    /// <example>
    /// DateTime finalTuesday = DateTime.Now.LastDayOfMonth(DayOfWeek.Tuesday);
    /// </example>
    /// <param name="dt" />Start date
    /// <param name="dayOfWeek" />The required day of week
    /// <returns />
    public static DateTime LastOfMonth(this DateTime dt, DayOfWeek dayOfWeek)
        {
        DateTime lastDayOfMonth = dt.LastOfMonth();
        return lastDayOfMonth.AddDays(lastDayOfMonth.DayOfWeek < dayOfWeek ? 
				dayOfWeek - lastDayOfMonth.DayOfWeek -  7 : 
				dayOfWeek - lastDayOfMonth.DayOfWeek) ;
        }
    /// <summary>
    /// Returns the next date which falls on the given day of the week
    /// </summary>
    /// <example>
    /// DateTime nextTuesday = DateTime.Now.NextDayOfWeek(DayOfWeek.Tuesday);
    /// </example>
    /// <param name="dt">Start date</param>
    /// <param name="dayOfWeek">The required day of week</param>
    public static DateTime NextDayOfWeek(this DateTime dt, DayOfWeek dayOfWeek)
        {
        int offsetDays = dayOfWeek - dt.DayOfWeek;
        return dt.AddDays(offsetDays > 0 ? offsetDays : offsetDays + 7).AtMidnight();
        }
    /// <summary>
    /// Returns the same day, at midnight
    /// </summary>
    /// <example>
    /// DateTime startOfDay = DateTime.Now.AtMidnight();
    /// </example>
    /// <param name="dt">Start date</param>
    public static DateTime AtMidnight(this DateTime dt)
        {
        return new DateTime(dt.Year, dt.Month, dt.Day, 0, 0, 0);
        }
    /// <summary>
    /// Returns the same day, at midday
    /// </summary>
    /// <example>
    /// DateTime startOfAfternoon = DateTime.Now.AtMidday();
    /// </example>
    /// <param name="dt">Start date</param>
    public static DateTime AtMidday(this DateTime dt)
        {
        return new DateTime(dt.Year, dt.Month, dt.Day, 12, 0, 0);
        }
    #endregion
    }
}

History

• V1.1 Added missing LastOfMonth method. Still not sure where it went...
• V1.2 6^th June 2012 LastOfMonth fixed: was giving incorrect result when the sought dayOfWeek enumeration value is more than the lastDayOfMonth.DayOfWeek enumeration value. Bug found and suggested fix by VJ Reddy[^] - My thanks to him, and my apologies for any inconvenience caused.