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Inconsistency when parsing DateTime in the same format

I have the following strings:
10/10/2021 00:00:00 and 18/11/2021 23:59:59
I have this code:
bool first = DateTime.TryParse("10/10/2021 00:00:00",
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out DateTime firstDate);
bool second = DateTime.TryParse("18/11/2021 23:59:59",
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out DateTime secondDate);
Console.WriteLine(firstDate + " --- " + secondDate);
The output is:
10/10/2021 12:00:00 AM --- 1/1/0001 12:00:00 AM
As you can see the second date is not properly parsed, even though it's in the same format. What is the reason for that and how can I fix it?

As you can see the second date is not properly parsed, even though it's in the same format.
Here my two cents.
Programming languages and frameworks are not smart enough to know which kind of format data you applied, specially for dates, times, numbers etc.. If you provide these data, you kinda have to provide the proper format as well so they can do their job. You "said" the same format, but you didn't apply any format in your code. So, as we humans, we know (at least you told us) but the computer don't know.
Let's look what TryParse(String, IFormatProvider, DateTimeStyles, DateTime) documentation says;
Converts the specified string representation of a date and time to its
DateTime equivalent using the specified culture-specific format
information and formatting style, and returns a value that indicates
whether the conversion succeeded.
You didn't supply format information you supplied IFormatProvider as InvariantCulture. So, what are these "culture specific formats"?
Well, most of them are returns with GetAllDateTimePatterns method (but not all of them) but be aware because documentation says;
You can use the custom format strings in the array returned by the
GetAllDateTimePatterns method in formatting operations. However, if
you do, the string representation of a date and time value returned in that formatting operation cannot always be parsed successfully by the
Parse and TryParse methods. Therefore, you cannot assume that the
custom format strings returned by the GetAllDateTimePatterns method
can be used to round-trip date and time values.
So, if you run;
CultureInfo.InvariantCulture.DateTimeFormat.GetAllDateTimePatterns().Dump();
*Dump is just an extension method of LINQPad by the way, it just outputs to the console.
You will get a lot of datetime patterns, but for our case, the important one is we get MM/dd/yyyy HH:mm:ss format for InvariantCulture.
As you can see, your 18/11/2021 23:59:59 data doesn't match with MM/dd/yyyy HH:mm:ss format because there is no 18th month on Gregorian calendar which is a DateTime instance belongs internally.
Your second parsing fails by the way, that's quite different just saying "the second date is not properly parsed" and this is how DateTime.TryParse method works as explained in the documentation;
When this method returns, contains the DateTime value equivalent to
the date and time contained in s, if the conversion succeeded, or
MinValue (which is 1/1/0001 12:00:00 AM) if the conversion failed. The conversion fails if the s
parameter is null, is an empty string (""), or does not contain a
valid string representation of a date and time.
So, best way to handle this to supply a "specific" format using with DateTime.TryParseExact method or one of its overloads like;
bool first = DateTime.TryParseExact("10/10/2021 00:00:00",
"dd/MM/yyyy HH:mm:ss",
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out DateTime firstDate);
bool second = DateTime.TryParseExact("18/11/2021 23:59:59",
"dd/MM/yyyy HH:mm:ss",
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out DateTime secondDate);

The default DateTime format is 'MM/dd/yyyy' and since you have the date in 'dd/MM/yyyy' format it gives you the output.
Maybe try changing the date format input as 11/18/2021 23:59:59

C# DateTime.TryParse makes me confused

I am using DateTime.TryParse method in my program to judge if a string value is DateTime, then I notice this:
DateTime.TryParse("9.08", out DateTime dt)
// true
DateTime.TryParse("2.52", out DateTime dt)
// false
Why would this happened ?

DateTime.TryParse is parsed information in the current DateTimeFormatInfo object, which is supplied implicitly by the current thread culture.
Because the DateTime.TryParse(String, DateTime) method tries to parse the string representation of a date and time using the formatting rules of the current culture, trying to parse a particular string across different cultures can either fail or return different results. If a specific date and time format will be parsed across different locales
In some cultures, DateTime separator is . rather than /.
On my computer.
DateTime.TryParse will Parse "9.08" be this year '09/08', 2018/09/08 is a valid datetime, so it's true.
DateTime.TryParse will Parse "2.52" be this year '02/52', but there isn't 52nd days on February, 2018/02/52 isn't a valid DateTime, so it will be false.
I would use DateTime.TryParseExact to Parse DateTime because you can set your CultureInfo and Parse DateTime string be parameters and ensure that conforms to your expected format.
DateTime.TryParseExact("09.08",
"MM.dd",
System.Globalization.CultureInfo.InvariantCulture,
System.Globalization.DateTimeStyles.None,
out dt);

As per DateTime.TryParse documentation:
returns a value that indicates whether the conversion succeeded.
As it couldn't parse "2.52" to any valid date, it returned false.

Except you are trying to understand every time string conversion of .NET
Otherwise, you should not able to answer "why exactly would that happened?"
DateTime.TryParse is just a simple condition handling to prevent you getting an error when you do
Convert.ToDateTime(dateString)
Thus, DateTime.TryParse = false means you should not do Convert.ToDateTime to that string.
Oppositely, if a string to DateTime.TryParse = true, then it means that string should match the expectation of .NET date string (it means .NET know how to convert that string to DateTime).

DateTime.ParseExact doesn't work for "yyyy-MM-dd HH:mm:ss"

I have System.DateTime.Now stored in a variable called StartDate (variable is of type DateTime?).
I am converting it to string as below:
string temp = StartDate.Value.ToString("yyyy-MM-dd HH:mm:ss")
Now the temp has value in "yyyy-MM-dd HH:mm:ss".
Now I want to again convert string temp to DateTime so, I am doing as below:
DateTime.ParseExact(temp,"yyyy-MM-dd HH:mm:ss",null)
but this doesn't work.
It always returns date in a same format as System.DatTime.Now. It should return value in "yyyy-MM-dd HH:mm:ss" format.

A value of type DateTime doesn't have a particular format. That date-and-time is stored as a couple of integer values (not one for each component, but much more compact).
When you parse a string, that date represented in that string is stored as those integers.
Only when you do a .ToString() (possibly with some format) you get a string representation of that date in a particular format.
When you hover over the DateTime value in the debugger, you see the result of a plain .ToString(), using some specific current culture.
The difference between Parse and ParseExact is that Parse tries several formats (and may fail on a string like "2/3/2017" - FEB 3rd or 2nd MAR?) while with ParseExact you supply one or more formats yourself. In both cases you end up with a value of the same DateTime type.

convert date string to datetime

I have date string in format dd-MMM-yyyy and want to convert this to datetime, when I use below code
DateTime.ParseExact("20-Oct-2012", "yyyy-MM-dd HH:mm tt", null)
it causing an error
String was not recognized as a valid DateTime.
When I modify above code
DateTime.ParseExact("20-Oct-2012", "dd-MMM-yyyy", null)
then I got date time in format (mm/dd/yyyy) : 10/20/2012 12:00:00 AM
But I need it should be converted in yyyy/mm/dd format. Please help me in this regard.

You should try this
DateTime.ParseExact("20-Oct-2012", "dd-MMM-yyyy", null).ToString("yyyy/mm/dd")
For further reading on formats Check This

You need to distinguish between two separate concerns: that of parsing your original string into an abstract DateTime representation, and that of converting the latter back into another string representation.
In your code, you're only tackling the former, and relying on the implicit ToString() method call (which uses the system's current locale) to convert it back to string. If you want to control the output format, you need to specify it explicitly:
// Convert from string in "dd-MMM-yyyy" format to DateTime.
DateTime dt = DateTime.ParseExact("20-Oct-2012", "dd-MMM-yyyy", null);
// Convert from DateTime to string in "yyyy/MM/dd" format.
string str = dt.ToString("yyyy/MM/dd");
Also note that the mm format specifier represents minutes; months are represented by MM.
Edit: 'Converted date contain value "10/20/2012 12:00:00 AM".' Be careful what you mean by that. The constructed DateTime value contains an abstract representation of the parsed date and time that is independent of any format.
However, in order to display it, you need to convert it back into some string representation. When you view the variable in the debugger (as you're presumably doing), Visual Studio automatically calls the parameterless ToString() method on the DateTime, which renders the date and time under the current culture (which, in your case, assumes the US culture).
To alter this behaviour such that it renders the date and time under a custom format, you need to explicitly call the ToString(string) overload (or one of the other overloads), as I've shown in the example above.

You could try this instead :
Convert.ToDateTime("20-Oct-2012").ToString("yyyy/MM/dd")
Hope this will help !!

Convert.DateTime

What Convert.DateTime will convert the date 7/25/2010 12:00:00 it's current format is(MM/dd/yyyy HH:mm:ss)?
When I convert this string format to date time I am getting the error "string was not recognized as valid DateTime"

None. Dates are not stored internally as a certain format.
If you want to parse a string into a date, use DateTime.ParseExact or DateTime.TryParseExact (the former will throw an exception if the conversion fails, the second uses an out parameter):
DateTime myDate = DateTime.ParseExact("7/25/2010 12:00:00",
"MM/dd/yyyy HH:mm:ss",
CultureInfo.InvariantCulture);
If you want to display a certain format, use ToString with the format string.
So, if you have a date object that represents midday of the 25th of July 2010 (doesn't matter how it is represented internally) and you want to format it with the format string "MM/dd/yyyy HH:mm:ss" you do the following:
string formattedDate = myDate.ToString("MM/dd/yyyy HH:mm:ss");

If you need to use Convert.DateTime, I'll assume you're working with a string you want to convert to a date. So you might try this:
DateTime date = Convert.DateTime("7/25/2010 12:00:00 am");
string formattedDateString = date.ToString("MM/dd/yyyy HH:mm:ss")
I'm making no assumptions as to why you'd want to do this, except that, well, you have your reasons.

DateTime.TryParse() or DateTime.Parse() will do the trick.
Edit: This is assuming that you are going from a string to a DateTime object.
Edit2: I just tested this with your input string, and I receive no error with DateTime.Parse