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How can I convert this date "03/01/2018 12:00 AM" to "2018-03-01" in C#?

I am using this code to convert a date 03/01/2018 12:00 AM to 2018-03-01 in C#:
DateTime startDate = DateTime.ParseExact(TextBox1.Text.ToString(),
"yyyy-mm-dd",
System.Globalization.CultureInfo.InvariantCulture);
but it throws an exception
String was not recognized as valid datetime

This Will Work Like A Charm
string bs = "03/01/2018 12:00 AM";
String startDate = DateTime.ParseExact(bs,"MM/dd/yyyy hh:mm tt",System.Globalization.CultureInfo.InvariantCulture).ToString("yyyy-MM-dd");
Console.WriteLine(startDate);
// Outputs 2018-03-01
You can verify the code here

A DateTime represents a particular point in time. ParseExact is a way of turning a string into a DateTime. By saying ParseExact with "yyyy-mm-dd" you are telling it that the string you are giving it begins with a four digit year, which it doesn't. Fix the format string that you are supplying so that the parse works.
Once you have the value in your DateTime variable, you can use ToString("yyyy-mm-dd") to turn that DateTime back into a string.

Remove AM from your Textbox and edit format string, then your sample code will work.
This line run successfully :
DateTime startDate = DateTime.ParseExact("03/01/2018 12:00", "MM/dd/yyyy hh:mm", System.Globalization.CultureInfo.InvariantCulture);

string inputDate = "03/01/2018 12:00 AM";
string outputDate = DateTime.Parse(inputDate).ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);

DateTime.Parse Not Parsing

I'm trying to return the date as "2015-06-18"
string strDate = DateTime.Now.ToString("yyyy-MM-dd");
DateTime newDate = DateTime.Parse(strDate);
This returns "2015/06/18 hh:mm:ss"
What am I missing?

If you want a particular output format, you can specify one yourself.
string strDate = DateTime.Now.ToString("yyyy-MM-dd");
DateTime newDate = DateTime.Parse(strDate);
string output = newDate.ToString("yyyy-MM-dd");
Console.WriteLine (output); // produces 2015-06-18 right now
The DateTime structure in .net always includes the time of day, and there is no built-in way to store only a date, so if you want to exclude it, you'll need to use the formatting options.

What you need is to format the datetime object.
newDate.ToString("yyyy-MM-dd") -> 2015-06-19

Why don't you just use the DateTime.Date property?
DateTime date1 = DateTime.Now;
Console.WriteLine(date1.ToString());
// Get date-only portion of date, without its time.
DateTime dateOnly = date1.Date;
// Display date using short date string.
Console.WriteLine(dateOnly.ToString("yyyy-MM-dd"));

Convert date time to string and back to date time

I'm having a troubles with converting strings to DateTime. Here is what I have. First I convert current date to string (this will be folder name).
string dateString = string.Format("{0:yyyy-MM-dd_HH-mm-ss}", DateTime.Now);
Output like this
2013-05-16_09-32-47
Then I create a folder. During program execution I get this folder and I need to convert it's name back to DateTime. Try to make it like this
DateTime directoreDate = DateTime.ParseExact(directory.Name, "0:yyyy-MM-dd_HH-mm-ss", CultureInfo.InvariantCulture);
But it throws FormatException. Can anybody tell me why this happening.

You are using the same composite format string that you used to format the original DateTime. This is not needed for ParseExact - drop the 0: from it:
DateTime directoreDate = DateTime.ParseExact(directory.Name,
"yyyy-MM-dd_HH-mm-ss",
CultureInfo.InvariantCulture);

Use
DateTime directoreDate = DateTime.ParseExact(directory.Name, "yyyy-MM-dd_HH-mm-ss", CultureInfo.InvariantCulture);

Remove 0: from DateTime.ParseExact, It was used as a place holder in string.Format().
Use as :
DateTime directoreDate = DateTime.ParseExact(directory.Name,
"yyyy-MM-dd_HH-mm-ss",
CultureInfo.InvariantCulture);

how to convert date with 'T' to/from string in C#

I used following functions to convert DateTime from/into string:
DATE_OBJ.ToString(DATE_FORMAT);
DateTime.ParseExact(Date_string, DATE_FORMAT, null);
Now I've got to work with follow format 2012-03-20T14:18:25.000+04:00
Which format should I use to convert it correctly to string and generate string like that from DateTime object?

You can go from DateTime to that format with
DateTime dt = new DateTime();
dt.ToString("o");
and from that format to DateTime with
DateTimeOffset.Parse(dateString);
Here is some more info on DateTime format:
http://www.dotnetperls.com/datetime-format

You are better of using DateTimeOffSet like:
string str = " 2012-03-20T14:18:25.000+04:00";
DateTimeOffset dto = DateTimeOffset.Parse(str);
//Get the date object from the string.
DateTime dtObject = dto.DateTime;
//Convert the DateTimeOffSet to string.
string newVal = dto.ToString("o");

You cannot do this from DateTime, as DateTime holds no TimeZone info.
This is close: string.Format("{0:s}", dt) will give 2012-03-20T14:18:25.
See: http://www.csharp-examples.net/string-format-datetime/
You could extend this to: string.Format("{0:s}.{0:fff}", dt), which will give 2012-03-20T14:18:25.000
But you better have a look at DateTimeOffset: DateTime vs DateTimeOffset
(Not advisable, but to fake it and still use DateTime: string.Format("{0:s}.{0:fff}+04:00", dt))

If that is a string you receive then you can split the string by T and use only the first part which is the Date component of the whole string and parse that.
ex:
string dateTimeAsString = "2012-03-20T14:18:25.000+04:00";
string dateComponent = dateTimeAsString.Splic('T')[0];
DateTime date = DateTime.ParseExact(dateComponent, "yyyy-MM-dd",null);

how to give format DateTime.Date?

DateTime dt = DateTime.Now
dt.Date is created to 31.10.2012 00:00:00 .it is created to dd.mm.yyyy format but i need dd/mm/yyyy. Can i use: return new DateTime(d.Year, d.Month, d.Day, 0, 0, 0); it will create to me dd/mm/yyyy solution?Please dont translate String.i need datetime...

The DateTime struct doesn't store any formatting information internally. If you want to output the DateTime instance as a formatted string, you just need to call ToString() with the proper format string:
var date = DateTime.Now;
var formattedString = date.ToString("dd/MM/yyyy", CultureInfo.InvariantCulture);
If you need more information on exactly which specifiers to use in your format string, check out:
MSDN - Custom Date and Time Format Strings

Just the way to convert to string, DateTime itself has no format:
var result = DateTime.Now.Date
.ToString("dd/MM/yyyy", CultureInfo.InvariantCulture);

var dt = DateTime.Now;
var stringDt = dt.Date.ToString("dd/MM/yyyy");

In you case you can simply use :
dt.ToString("dd/MM/yyyy");
Anyway there al the string format you can use with DateTime : Here.

System.DateTime does not have any format. You can view its string representation in format.
Try this
Console.WriteLine(DateTime.Now.Date.ToString("dd'/'MM'/'yyyy"));

DateTime, numeric types and most other types do not store their values in a formatted way. Rather they store their data using a binary representation. If you want to display this data to the user, you must convert it to a string. This conversion involves formatting the data.
string formattedDate = DateTime.Now.ToString("dd/MM/yyyy",
CultureInfo.InvariantCulture);
Or
Console.WriteLine("Date = {0:dd/MM/yyyy}", DateTime.Now);
Console.WriteLine converts the date into a string in order to write it to the console.

DateTime structure always has the Date and Time stored in it. If you need to extract the date alone as text you can do the following.
var date = DateTime.Now.ToString("d");
Console.WriteLine(date);
This will print the date as in the format as specified by the culture set in the system. The list of standard datetime format strings supported by dotnet framework can be found here