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DateTime.ParseExact: String was not recognized as a valid DateTime

I am trying to get the date (& datetime) from the url and then convert it into its proper format before storing it in the db.
var reqDate = Request.QueryString["StartDate"];
//at this point I have reqDate: 05/15/2018 00:00:00
reqDate = reqDate.Substring(0, reqDate.IndexOf(" ") + 1);
//after stripping off the time part I have: 05/15/2018
timingRequest.ReqDate = DateTime.ParseExact(reqDate, "MM/dd/yyyy", CultureInfo.InvariantCulture);
//but this throws the exception
URL:
Same is the case with startDateTime
var reqDateTime = Request.QueryString["startDateTime"];
timingRequest.IgnoreEntry = DateTime.ParseExact(reqDateTime, "dd/MM/yyyy hh:mm tt", CultureInfo.InvariantCulture);

In your first scenario, No need to add +1 after reading indexOf(" "). +1 adding extra space to date
//Lets take date in string is "05/15/2018 00:00:00"
Console.WriteLine(s.Substring(0, reqDate.IndexOf(" ")+1)); /*This will print "05/15/2018 " WITH EXTRA SPACE*/
Correct way is s.Substring(0, s.IndexOf(" "))
In second scenario, use date format like HH:mm:ss instead of HH:mm tt
//Here use "hh:mm:ss" instead of "hh:mm tt"
DateTime dateTime = DateTime.ParseExact(reqDateTime, "dd/MM/yyyy hh:mm:ss", CultureInfo.InvariantCulture);
Elegant approach would be:
#Credit Stephen Muecke
After looking at your URL, you can write a method having parameters like,
public ActionResult Create(int empId, int attID, DateTime startDate, DateTime startDateTime)
{
/*Do your work here, DefaultModelBinder will take care of parameters*/
}

First Example fix:
var reqDate = Request.QueryString["StartDate"];
reqDate = reqDate.Substring(0, reqDate.IndexOf(" "));
timingRequest.ReqDate = DateTime.ParseExact(reqDate, "MM/dd/yyyy", CultureInfo.InvariantCulture);
Second Example Fix:
var reqDateTime = Request.QueryString["startDateTime"];
timingRequest.IgnoreEntry = DateTime.ParseExact(reqDateTime, "dd/MM/yyyy HH:mm:ss", CultureInfo.InvariantCulture);

String was not recognized as a valid DateTime. Throws an Exception

I'm trying to convert current date to a specified format.
DateTime date = DateTime.ParseExact(DateTime.Now.ToString(), "yyyy-MM-dd HH:mm:ss.fff",
CultureInfo.InvariantCulture,
DateTimeStyles.None);
I'm receiving the following exception.
String was not recognized as a valid DateTime.
My local TimeZone is (UTC+10:00)Melbourne.
What am I doing wrong here?

Your code (even if it worked), would do nothing. It would simply serialize and deserialize the date. I believe you're looking for this:
string date = DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff");
It doesn't work because DateTime.Now.ToString() is giving a string like (I happen to be in the same timezone, and presumably have the same culture as you):
14/01/2016 3:54:01 PM
Which is of the format:
dd/MM/yyyy h:mm:ss tt
Which does not match the format you're using: yyyy-MM-dd HH:mm:ss.fff

Try this:
string fm = "yyyy-MM-dd HH:mm:ss.fff";
string str = DateTime.Now.ToString(fm, CultureInfo.InvariantCulture);
DateTime dt = DateTime.ParseExact(str, fm, CultureInfo.InvariantCulture);
EDIT:
A better way to achieve the date in the format would be like
DateTime now = DateTime.Now;
CultureInfo culture = new CultureInfo("en-AU"); //Melbourne
Thread.CurrentThread.CurrentCulture = culture;
Console.WriteLine(now.ToString("yyyy-MM-ddTHH:mm:ss.fff"));
IDEONE DEMO

DateTime.ParseExact with format string returns weird value

I try to parse a date providing a specified format with the following code :
var date = "30/06/2014";
var ret2 = DateTime.ParseExact(date, "dd/mm/yyyy", null);
var ret3 = DateTime.ParseExact(date, "dd/mm/yyyy", CultureInfo.CurrentCulture);
var ret4 = DateTime.ParseExact(date, "dd/mm/yyyy", CultureInfo.InvariantCulture);
Console.WriteLine(ret2);
Console.WriteLine(ret3);
Console.WriteLine(ret4);
-----OUTPUT-----
30/01/2014 00:06:00
30/01/2014 00:06:00
30/01/2014 00:06:00
Could someone explain me why this code doesn't return the 30/06/2014 00:00:00 value I am expecting ?

mm is the placeholder for minutes. You should use MM for months

mm means minutes, not months. Try using MM instead.

As mentioned, mm represents Minutes - note that the minutes of each DateTime is 06:00 instead of the default 00:00, and 1 is the default for a month(since there is no 0 month).
See MSDN on Standard and Custom DateTime format strings for more info, as well as more helpful placeholders.

the code used the wrong format string
var ret2 = DateTime.ParseExact(date, "dd/mm/yyyy", null);
var ret3 = DateTime.ParseExact(date, "dd/mm/yyyy", CultureInfo.CurrentCulture);
var ret4 = DateTime.ParseExact(date, "dd/mm/yyyy", CultureInfo.InvariantCulture);
mm is placeholder for minutes.
to get month, use MM in the format string
var ret2 = DateTime.ParseExact(date, "dd/MM/yyyy", null);
var ret3 = DateTime.ParseExact(date, "dd/MM/yyyy", CultureInfo.CurrentCulture);
var ret4 = DateTime.ParseExact(date, "dd/MM/yyyy", CultureInfo.InvariantCulture);
check out the example on MSDN https://msdn.microsoft.com/en-us/library/w2sa9yss(v=vs.110).aspx

Odd results with parsing a Datetime c#

I have the following code:
IFormatProvider culture = new System.Globalization.CultureInfo("es-ES", true);
date = DateTime.ParseExact(_date, "yyyy-MM-dd hh:mm", culture);
for _date = "2012-11-17 15:00"
it throws an exception
but for _date = "2012-11-17 10:00" works
Anyone can tell me what I'm doing wrong?

use HH instead of hh
date = DateTime.ParseExact(_date, "yyyy-MM-dd HH:mm", culture);
HH is for 24-hr
hh is for 12-hr

Convert Date String to DateTime

I have to convert a date string to a DateTime object as follows
tmpdate = "27-Apr 14:53";
TheDate = DateTime.ParseExact(tmpdate, "DD-MMM HH:mm", CultureInfo.InvariantCulture);
I keep getting exceptions about the string not being a valid date time. I've tried adding in the year as well with no luck. Any suggestions?

Try dd-MMM HH:mm - note lowercased dd.

try with little d :
TheDate = DateTime.ParseExact(tmpdate, "dd-MMM HH:mm", CultureInfo.InvariantCulture);

string tmpdate = "27-Apr 14:53";
DateTime TheDate = DateTime.ParseExact(tmpdate, "dd-MMM HH:mm", System.Globalization.CultureInfo.InvariantCulture);