C#

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace DateValidator
{
	public class frmDateInput : System.Windows.Forms.Form
	{
		private System.Windows.Forms.TextBox txtDate;
		private System.Windows.Forms.Label lblDateFormat;
		private System.Windows.Forms.Button btnCheckDate;
		private System.Windows.Forms.Label lblDateStatus;
		private System.Windows.Forms.Label lblDateStatusTag;
		/// Required designer variable.
		private System.ComponentModel.Container components = null;

		public frmDateInput()
		{
			// Required for Windows Form Designer support
			InitializeComponent();
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#region Windows Form Designer generated code
		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			this.txtDate = new System.Windows.Forms.TextBox();
			this.btnCheckDate = new System.Windows.Forms.Button();
			this.lblDateFormat = new System.Windows.Forms.Label();
			this.lblDateStatus = new System.Windows.Forms.Label();
			this.lblDateStatusTag = new System.Windows.Forms.Label();
			this.SuspendLayout();
			// 
			// txtDate
			// 
			this.txtDate.Location = new System.Drawing.Point(16, 17);
			this.txtDate.Name = "txtDate";
			this.txtDate.Size = new System.Drawing.Size(176, 20);
			this.txtDate.TabIndex = 0;
			this.txtDate.Text = "";
			// 
			// btnCheckDate
			// 
			this.btnCheckDate.Location = new System.Drawing.Point(208, 16);
			this.btnCheckDate.Name = "btnCheckDate";
			this.btnCheckDate.Size = new System.Drawing.Size(96, 23);
			this.btnCheckDate.TabIndex = 2;
			this.btnCheckDate.Text = "Check Date";
			this.btnCheckDate.Click += new System.EventHandler(this.btnCheckDate_Click);
			// 
			// lblDateFormat
			// 
			this.lblDateFormat.Location = new System.Drawing.Point(16, 44);
			this.lblDateFormat.Name = "lblDateFormat";
			this.lblDateFormat.Size = new System.Drawing.Size(100, 16);
			this.lblDateFormat.TabIndex = 3;
			this.lblDateFormat.Text = "MM/DD/YYYY";
			// 
			// lblDateStatus
			// 
			this.lblDateStatus.Location = new System.Drawing.Point(88, 72);
			this.lblDateStatus.Name = "lblDateStatus";
			this.lblDateStatus.Size = new System.Drawing.Size(216, 16);
			this.lblDateStatus.TabIndex = 4;
			// 
			// lblDateStatusTag
			// 
			this.lblDateStatusTag.Location = new System.Drawing.Point(16, 72);
			this.lblDateStatusTag.Name = "lblDateStatusTag";
			this.lblDateStatusTag.Size = new System.Drawing.Size(72, 16);
			this.lblDateStatusTag.TabIndex = 5;
			this.lblDateStatusTag.Text = "Date Status:";
			// 
			// frmDateInput
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(320, 94);
			this.Controls.AddRange(new System.Windows.Forms.Control[] {
																		  this.lblDateStatusTag,
																		  this.lblDateStatus,
																		  this.lblDateFormat,
																		  this.btnCheckDate,
																		  this.txtDate});
			this.Name = "frmDateInput";
			this.Text = "Date Validator";
			this.ResumeLayout(false);

		}
		#endregion


		[STAThread]
		static void Main() 
		{
			Application.Run(new frmDateInput());
		}

		private void btnCheckDate_Click(object sender, System.EventArgs e)
		{
			if(txtDate.Text != string.Empty)
			{
				int retStatus = DateValidator.parseDate(txtDate.Text);
				///		1 = Date is valid
				///		0 = Date is not valid, reason unknown
				///	   -1 = Month is out of range
				///	   -2 = Day is out of range
				///	   -3 = Year is out of range
				///	   -4 = The whole date was not passed
				switch(retStatus)
				{
					case 1:
						lblDateStatus.Text = "This date is valid!";
					break;
					case 0:
						lblDateStatus.Text = "This date is NOT valid, reason unknown!";
					break;
					case -1:
						lblDateStatus.Text = "The month is not correct!";
					break;
					case -2:
						lblDateStatus.Text = "The day is not correct!";
					break;
					case -3:
						lblDateStatus.Text = "The year is not correct";
					break;
					case -4:
						lblDateStatus.Text = "The date string tested was not complete!";
					break;
				}
				
			}
			else
			{
				MessageBox.Show("A date is required!", "Error!");
			}
		}
	}
}

using System;

namespace DateValidator
{
	/// <summary>
	/// DateValidator Class
	/// This class takes one param:
	///		1. string type = date
	///	This class returns one of the following values:
	///		1 = Date is valid
	///		0 = Date is not valid, reason unknown
	///	   -1 = Month is out of range
	///	   -2 = Day is out of range
	///	   -3 = Year is out of range
	///	   -4 = The whole date was not passed
	/// </summary>
	public class DateValidator
	{
		public static int parseDate(string txtDate)
		{
			//We need a start year and an end year to test against
			int theMinYear = 2000;
			int theMaxYear = 2020;
			// Build our date array out of the string
			string[] dateElements = txtDate.Split(new Char[] {'/'});
			// Check to make sure we have 3 date elements
			if(dateElements.Length == 3)
			{
				// Create the var's that we will use to test with
				int theMonth = Convert.ToInt32(dateElements[0],10);
				int theDay = Convert.ToInt32(dateElements[1],10);
				int theYear = Convert.ToInt32(dateElements[2],10);
				// First we will test the month
				if(theMonth > 0 && theMonth < 13)
				{
					if(theYear > theMinYear && theYear < theMaxYear)
					{
						switch(theMonth)
						{
							case 1: // January
								if(theDay > 0 && theDay < 32)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 2: // February
								if((theYear % 4 == 0 && theYear % 100 != 0) || theYear % 400 == 0)
								{
									if(theDay > 0 && theDay < 30)
									{
										return 1;
									}
									else
									{
										return -2;
									}
								}
								else
								{
									if(theDay > 0 && theDay < 29)
									{
										return 1;
									}
									else
									{
										return -2;
									}
								}
							break;
							
							case 3: // March
								if(theDay > 0 && theDay < 32)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 4: // April
								if(theDay > 0 && theDay < 31)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 5: // May
								if(theDay > 0 && theDay < 32)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 6: // June
								if(theDay > 0 && theDay < 31)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 7: // July
								if(theDay > 0 && theDay < 32)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 8: // August
								if(theDay > 0 && theDay < 32)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 9: // September
								if(theDay > 0 && theDay < 31)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 10: // October
								if(theDay > 0 && theDay < 32)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 11: // November
								if(theDay > 0 && theDay < 31)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							case 12: // December
								if(theDay > 0 && theDay < 32)
								{
									return 1;
								}
								else
								{
									return -2;
								}
							break;
							
							default:
								return 0;
							break;
						}
					}
					else // If the year is not in our range we send back -3
					{
						return -3;
					}
				}
				else // If the month fails we send back -1 status
				{
					return -1;
				}
			} 
			else
			{
				return -4;
			}
		}
	}
}