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How to convert the "time" from DateTime into int?

I have a DataGrid which contains a few values that are in hours and I wanted to know:
How to get ONLY the time from my DataGrid and convert it into an int (or double) variable.
My goal is to do a few operations with my DataGrid time values, like to add numbers into it
EXAMPLE:
Using my "dataGridView1.Rows[1].Cells[2].Value.ToString();" It'll show a DateTime value (which is inside my DataGrid), with this value, I wanna filter ONLY the time from this and convert it into an int
the part of my code which I wanna "capture" the time:
txtAtiv.Text = dataGridView1.Rows[0].Cells[1].Value.ToString();
string value = dataGridView1.Rows[0].Cells[2].Value.ToString();
lblLeft.Text = value.Split(' ')[1];
I wanna get the "value" (which is a DateTime value from the DataGrid) and convert it into an int.
note:
- The date for me in my dataGrid it's not relevant, I only have to pick the time (and yes, I know that I can't "split" DateTime to do them separately)

If you are willing to be limited to millisecond resolution, then this is fairly easy.
Given a date/time that you want to get the time part from as an int, you can get the number of milliseconds since midnight, like so:
DateTime dateTime = DateTime.Now;
int timeMsSinceMidnight = (int)dateTime.TimeOfDay.TotalMilliseconds;
If you want to reconstitute the original date and time from this, you need the original date and the time since midnight in milliseconds:
DateTime date = dateTime.Date; // Midnight.
DateTime restoredTime = date.AddMilliseconds(timeMsSinceMidnight);
Test program:
DateTime dateTime = DateTime.Now;
Console.WriteLine("Original date/time: " + dateTime );
int timeMsSinceMidnight = (int)dateTime.TimeOfDay.TotalMilliseconds;
DateTime date = dateTime.Date; // Midnight.
DateTime restoredTime = date.AddMilliseconds(timeMsSinceMidnight);
Console.WriteLine("Restored date/time: " + restoredTime);
The value returned from time.TimeOfDay is of type TimeSpan, which is convenient for storing time-of-day values.
If you want to turn your "milliseconds since midnight" back into a TimeSpan, you just do this:
var timeSpan = TimeSpan.FromMilliseconds(timeMsSinceMidnight);

First step is to convert string to DateTime. Use DateTime.TryParse(string value, out DateTime dt). Then as Mathew Watson rightly suggested, get the value of variable dt converted to milliseconds using dt.TimeOfDay.TotalMilliseconds. It is also possible to convert the span in TotalSeconds or TotalMinutes if it suits your requirement.
Try to avoid calling ToString() method directly before checking if cell value is null. If I want to avoid the check, I would make compiler to do it by using something like : Rows[3].Cells[2].Value + "" instead of Value.ToString().

Mixing Mathew's and Mukesh Adhvaryu's answers, I got into this one, and it fits perfectly on what I need, thank you guys for your support!
txtAtiv.Text = dataGridView1.Rows[0].Cells[1].Value + "";
string value = dataGridView1.Rows[0].Cells[2].Value + "";
lblLeft.Text = value.Split(' ')[1];
textStatus.Text = "";
DateTime timeConvert;
DateTime.TryParse(value, out timeConvert);
double time;
time = timeConvert.TimeOfDay.TotalMilliseconds;
var timeSpan = TimeSpan.FromMilliseconds(time);
lblSoma.Text = timeSpan.ToString();

static void Main(string[] args)
{
string time1 = "11:15 AM";
string time2 = "11:15 PM";
var t1 = ConvertTimeToInt(time1);
var t2 = ConvertTimeToInt(time2);
Console.WriteLine("{0}", t1);
Console.WriteLine("{0}", t2);
Console.WriteLine("{0:dd/MM/yyyy hh:mm tt}", ConvertIntToTime(t1));
Console.WriteLine("{0:dd/MM/yyyy hh:mm tt}", ConvertIntToTime(t2));
Console.ReadLine();
}
static long ConvertTimeToInt(string input)
{
var date = DateTime.ParseExact(input, "hh:mm tt", CultureInfo.InvariantCulture);
TimeSpan span = date.TimeOfDay;
Console.WriteLine("{0:dd/MM/yyyy hh:mm tt}", date);
return span.Ticks;
}
static DateTime ConvertIntToTime(long input)
{
TimeSpan span = TimeSpan.FromTicks(input);
var date = new DateTime(span.Ticks);
Console.WriteLine("{0:dd/MM/yyyy hh:mm tt}", date);
return date;
}

Remove '0' (Number) from DateTime Month and Day (String)

Can anybody tell me the best approach or solution on how I would do the following?
I have a DateTime (as String) in following format:
string test = "21.12.2013";
How could I now remove all zero's from the month and day but still 'keep' the DateTime Logic:
//Example 1
string input = "06.10.2013" // 6th October
string output = "6.10.2013" //only remove '0' from the day
//Example 2
string input = "01.09.2012" // 1st September
string output = "1.9.2012" //remove from month and day
//Example 3
string input = "20.10.2011" // 20th October
string output = "20.10.2011" //should (must) stay!
I can also parse to DateTime if that would be make it easier but yeah I hope you got my idea...
Any help appreciated!

Parsing your string into DateTime and getting it back to string using ToString with desired patter seems to be the easiest way to go:
public static string GetRidOfZeros(string input)
{
var dt = DateTime.ParseExact(input, "dd.MM.yyyy", CultureInfo.InvariantCulture);
return dt.ToString("d.M.yyyy", CultureInfo.InvariantCulture);
}
Little testing, with your sample data:
var inputs = new List<string> { "06.10.2013", "01.09.2012", "20.10.2011" };
var outputs = new List<string> { "6.10.2013", "1.9.2012","20.10.2011" };
if(outputs.SequenceEqual(inputs.Select(d => GetRidOfZeros(d))))
Console.WriteLine("Output is OK");
else
Console.WriteLine("Collections does not match.");
Prints Output is OK.

DateTime.Parse(input).ToString("d.M.yyyy")

As you said, parsing to DateTime first would probably make things easier, since then you can just use:
myDateTime.ToString("d.M.yyyy");

When you parse it you can use ToString to format it any way you like:
var date = "06.10.2013";
DateTime parsed = DateTime.ParseExact(date, "dd.MM.yyyy", CultureInfo.InvariantCulture);
var noZerosHere = parsed.ToString("d.MM.yyyy");

A decent "catch-all" method (that will work not just on DateTime but ANY kind of string) would be to split the string up, take out leading zeroes and then put the pieces back together again.
string input = "01.09.2012";
string[] values = input.Split(".");
string[] modifiedValues = values.Select(x => x.TrimStart('0');
string output = String.Join(".", modifiedValues);
You can adjust the delimiters for different representations of DateTime, e.g. those that use slashes (01/09/2012) or are written in a different order.

C# custom time format

Subtraction two time format ,
string _time_One = "08:30" ;
string _time_Two = "08:35" ;
string _timeInterval = ( DateTime.Parse(_time_One) - DateTime.Parse(_time_Two) ).Minutes.ToString();
It give me the result 5 , but I want to show likes this format 00:05.
Kindly show me how to format it . Thanks in advance !

Subtracting two DateTime gives you a TimeSpan which has it's own formatting support using the ToString method.
For example:
DateTime now = DateTime.Now;
DateTime later = now.AddMinutes(10);
TimeSpan span = later - now;
string result = span.ToString(#"hh\:mm");
You can read more here on MSDN.

Try this:
string _time_One = "08:30";
string _time_Two = "08:35";
var span = (DateTime.Parse(_time_One) - DateTime.Parse(_time_Two));
string _timeInterval = string.Format("{0:hh\\:mm}", span);
For reference: Custom TimeSpan Format Strings.

#Lloyd is right here, but to clarify this for your case:
string _time_One = "08:30" ;
string _time_Two = "08:35" ;
TimeSpan ts = DateTime.Parse(_time_One) - DateTime.Parse(_time_Two);
MessageBox.Show(String.Format("Time: {0:00}:{1:00}", ts.Hours, ts.Minutes));
I hope this helps.

string _time_One = "08:30";
string _time_Two = "08:35";
string _timeInterval = (DateTime.Parse(_time_One) - DateTime.Parse(_time_Two)).Duration().ToString();
result=>00:05:00

Parse date time from string of format ddMMMyyyy hhmm (with Month-Name)

I am kind of stuck with an issue where I am unable to to parse the date and time from a string, which I am reading from a text file. The string I am getting is in following format:
05SEP1998 2400
and I am trying to parse the string through the following code:
string dateTimeStr = "05SEP1998 2400"
var provider = CultureInfo.InvariantCulture;
const string Format = "ddMMMyyyy hhmm";
var dateTime = DateTime.ParseExact(dateTimeStr, Format, provider);
But while parsing, the above code throws a FormatException:
String was not recognized as a valid DateTime.
Could anybody please help me fixing this issue?

hh is 12 hour, HH is 24 hour. However, it must be in the range 0-23, not 24. If you can't easily change how those date strings are generated, you can parse it manually:
string dateTimeStr = "05SEP1998 2400";
var provider = CultureInfo.InvariantCulture;
const string Format = "ddMMMyyyy HHmm";
int HourPos = Format.IndexOf("HH");
var hour = dateTimeStr.Substring(HourPos, 2);
bool addDay = hour == "24";
if (addDay)
dateTimeStr = dateTimeStr.Substring(0, HourPos) + "00" + dateTimeStr.Substring(HourPos + 2);
var dateTime = DateTime.ParseExact(dateTimeStr, Format, provider);
if (addDay)
dateTime += TimeSpan.FromHours(24);
Note that this will throw exceptions if dateTimeStr doesn't have the right number of characters. You might want to handle that better.

There are 24 hours in a day. But while writing we say its from 0-23. It is giving exception on hours format.
How I found out?
I tried creating a DateTime object from your string like
DateTime dt = new DateTime(1998, 9, 5, 24, 0, 0);
It gave error on Hours that Hour. minute and second parameters descrive an un-representable DateTime

String was not recognized as a valid DateTime

I want to add a date in session (date1) like this:
Session["DateLesson"] = date1.ToString("dd.MM.yyyy");
Now from the session I want take this value:
var asd = Session["DateLesson"];
/*asd = "20.04.2012"*/
var datelesson = DateTime.Parse((string) asd);
And it gives me this exception:
FormatException not recognized as a valid DateTime

A period is not a valid/standard separator character in most locales. You'll need to use DateTime.ParseExact() in combination with a format string to tell the function how to read it. More importantly, if reading it back to a datetime is your main goal, why not just put the datetime in the session as is? That seems way more efficient, easier, and more maintainable to me.

Why persist your date as a string?
You could simply store it like this:
Session["DateLesson"] = date1;
And then retrieve it like this:
var datelesson = (DateTime)Session["DateLesson"];

string value = "20.04.2012";
DateTime datetime = DateTime.ParseExact(value, "dd.MM.yyyy", null);
This will return 4/20/2012 12:00:00:00 AM

Don't keep value as a string but as an object of the initial type:
public DateTime? DateLesson
{
get
{
DateTime? dateTime = Session["DateLesson"] as DateTime?;
if (dateTime.HasValue) // not null
{
// use dateTime.Value
}
}
set
{
Session["DateLesson"] = value;
}
}