.NET DateTime Format String

You can display a DateTime with custom format using DateTime.ToString() or String.Format() methods in .NET. The custom format string follows a set of naming convention such as y for year, M for month, d for day, h for hour 12, m for minute, s for second, and z for time zone. Let's create a simple ASP.NET page to exam the DateTime format string:

    public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            string dtString = string.Empty;
            DateTime dt = DateTime.Now;
            StringBuilder sb = new StringBuilder();
            sb.Append("y yy yyy yyyy \t--- (Year) "  + dt.ToString("y yy yyy yyyy") + "<br>");
            sb.Append("M MM MMM MMMM \t--- (Month) "  + dt.ToString("M MM MMM MMMM") + "<br>");
            sb.Append("d dd ddd dddd \t--- (Day) "  + dt.ToString("d dd ddd dddd") + "<br>");
            sb.Append("h hh H HH \t--- (Hour) "  + dt.ToString("h hh H HH") + "<br>");
            sb.Append("m mm \t\t--- (Minute) "  + dt.ToString("m mm") + "<br>");
            sb.Append("z zz zzz \t--- (Zone) "  + dt.ToString("z zz zzz") + "<br>");

            sb.Append("d \t--- " + dt.ToString("d") + "<br>");
            sb.Append("D \t--- " + dt.ToString("D") + "<br>");
            sb.Append("f \t--- " + dt.ToString("f") + "<br>");
            sb.Append("F \t--- " + dt.ToString("F") + "<br>");
            sb.Append("g \t--- " + dt.ToString("g") + "<br>");
            sb.Append("G \t--- " + dt.ToString("G") + "<br>");
            sb.Append("m \t--- " + dt.ToString("m") + "<br>");
            sb.Append("r \t--- " + dt.ToString("r") + "<br>");
            sb.Append("s \t--- " + dt.ToString("s") + "<br>");
            sb.Append("u \t--- " + dt.ToString("u") + "<br>");
            sb.Append("U \t--- " + dt.ToString("U") + "<br>");
            sb.Append("y \t--- " + dt.ToString("y") + "<br>");

            sb.Append("yyyy-MM-dd \t\t--- " + dt.ToString("yyyy-MM-dd") + "<br>");
            sb.Append("yyyy-MM-dd HH:mm \t--- " + dt.ToString("yyyy-MM-dd HH:mm") + "<br>");
            sb.Append("yyyy-MM-dd HH:mm:ss \t--- " + dt.ToString("yyyy-MM-dd HH:mm:ss") + "<br>");
            sb.Append("yyyy-MM-dd HH:mm tt \t--- " + dt.ToString("yyyy-MM-dd HH:mm tt") + "<br>");
            sb.Append("yyyy-MM-dd H:mm \t--- " + dt.ToString("yyyy-MM-dd H:mm") + "<br>");
            sb.Append("yyyy-MM-dd h:mm \t--- " + dt.ToString("yyyy-MM-dd h:mm") + "<br>");
            sb.Append("ddd, dd MMM yyyy HH:mm \t--- " + dt.ToString("ddd, dd MMM yyyy HH:mm") + "<br>");
            sb.Append("dddd, dd MMMM yyyy HH:mm \t--- " + dt.ToString("dddd, dd MMMM yyyy HH:mm") + "<br>");

            lblDateTimeFormat.Text = sb.ToString();
        }
    }

Result:

y yy yyy yyyy  --- (Year) 6 06 2006 2006
M MM MMM MMMM  --- (Month) 1 01 Jan January
d dd ddd dddd  --- (Day) 18 18 Wed Wednesday
h hh H HH      --- (Hour) 9 09 21 21
m mm           --- (Minute) 12 12
z zz zzz       --- (Zone) -5 -05 -05:00
d  --- 1/18/2006
D  --- Wednesday, January 18, 2006
f  --- Wednesday, January 18, 2006 9:12 PM
F  --- Wednesday, January 18, 2006 9:12:36 PM
g  --- 1/18/2006 9:12 PM
G  --- 1/18/2006 9:12:36 PM
m  --- January 18
r  --- Wed, 18 Jan 2006 21:12:36 GMT
s  --- 2006-01-18T21:12:36
u  --- 2006-01-18 21:12:36Z
U  --- Thursday, January 19, 2006 2:12:36 AM
y  --- January, 2006
yyyy-MM-dd           --- 2006-01-18
yyyy-MM-dd HH:mm     --- 2006-01-18 21:12
yyyy-MM-dd HH:mm:ss  --- 2006-01-18 21:12:36
yyyy-MM-dd HH:mm tt  --- 2006-01-18 21:12 PM
yyyy-MM-dd H:mm      --- 2006-01-18 21:12
yyyy-MM-dd h:mm      --- 2006-01-18 9:12
ddd, dd MMM yyyy HH:mm    --- Wed, 18 Jan 2006 21:12
dddd, dd MMMM yyyy HH:mm  --- Wednesday, 18 January 2006 21:12

In reality a DateTime value is often passed through with a formatted string, and is converted back to DateTime at some point. The safest way for parsing a DateTime string is using the TryParseExact method:

string dtString = DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss");
DateTime curDate;
DateTime.TryParseExact(dtString, "yyyy-MM-dd HH:mm:ss", DateTimeFormatInfo.InvariantInfo, 
                 DateTimeStyles.None, out curDate);