string date = DateTime.Now.ToString("yyyyMMdd hh:mm:ss");
Console.WriteLine(date);
//20120628 03:23:36

------解决方案--------------------
探讨


DateTime.Now.ToString("yyyyMMdd HH:mm:ss");


------解决方案--------------------
楼上的都对了，送个完整的给你

DateTime dt = DateTime.Now;  
Label1.Text = dt.ToString();//2005-11-5 13:21:25  
Label2.Text = dt.ToFileTime().ToString();//127756416859912816  
Label3.Text = dt.ToFileTimeUtc().ToString();//127756704859912816  
Label4.Text = dt.ToLocalTime().ToString();//2005-11-5 21:21:25  
Label5.Text = dt.ToLongDateString().ToString();//2005年11月5日  
Label6.Text = dt.ToLongTimeString().ToString();//13:21:25  
Label7.Text = dt.ToOADate().ToString();//38661.5565508218  
Label8.Text = dt.ToShortDateString().ToString();//2005-11-5  
Label9.Text = dt.ToShortTimeString().ToString();//13:21  
Label10.Text = dt.ToUniversalTime().ToString();//2005-11-5 5:21:25  
?2005-11-5 13:30:28.4412864  
Label1.Text = dt.Year.ToString();//2005  
Label2.Text = dt.Date.ToString();//2005-11-5 0:00:00  
Label3.Text = dt.DayOfWeek.ToString();//Saturday  
Label4.Text = dt.DayOfYear.ToString();//309  
Label5.Text = dt.Hour.ToString();//13  
Label6.Text = dt.Millisecond.ToString();//441  
Label7.Text = dt.Minute.ToString();//30  
Label8.Text = dt.Month.ToString();//11  
Label9.Text = dt.Second.ToString();//28  
Label10.Text = dt.Ticks.ToString();//632667942284412864  
Label11.Text = dt.TimeOfDay.ToString();//13:30:28.4412864  
Label1.Text = dt.ToString();//2005-11-5 13:47:04  
Label2.Text = dt.AddYears(1).ToString();//2006-11-5 13:47:04  
Label3.Text = dt.AddDays(1.1).ToString();//2005-11-6 16:11:04  
Label4.Text = dt.AddHours(1.1).ToString();//2005-11-5 14:53:04  
Label5.Text = dt.AddMilliseconds(1.1).ToString();//2005-11-5 13:47:04  
Label6.Text = dt.AddMonths(1).ToString();//2005-12-5 13:47:04  
Label7.Text = dt.AddSeconds(1.1).ToString();//2005-11-5 13:47:05  
Label8.Text = dt.AddMinutes(1.1).ToString();//2005-11-5 13:48:10  
Label9.Text = dt.AddTicks(1000).ToString();//2005-11-5 13:47:04  
Label10.Text = dt.CompareTo(dt).ToString();//0  
//Label11.Text = dt.Add(?).ToString();//问号为一个时间段  
Label1.Text = dt.Equals("2005-11-6 16:11:04").ToString();//False  
Label2.Text = dt.Equals(dt).ToString();//True  
Label3.Text = dt.GetHashCode().ToString();//1474088234  
Label4.Text = dt.GetType().ToString();//System.DateTime  
Label5.Text = dt.GetTypeCode().ToString();//DateTime