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Date was not recognized as a valid DateTime

kinda got an issue that I cant solve right now.
I've got a discord bot running on my raspberry pi, which has a system for automated messages that are sent after a certain amount of time, or an exact date, has passed.
My code works on Windows when debugging there, but the console throws a warning on Linux when running the published project.
The date is taken from a table in my MySQL database and put into a DataTable. The code that grabs the date from the DataRow is:
DateTime datetime = DateTime.ParseExact(row["datetime"].ToString(), "dd.MM.yyyy HH:mm:ss", CultureInfo.InvariantCulture);
Why is it happening? No matter how I format the string (dots, dashes or slashes), the warning persists. The messages are not sent.
I even tried removing invisible whitespaces with regex, doesnt work either.
(The regex in question, though I scrapped it since it yielded no fruit anyways)
Regex.Replace($"{row["datetime"].ToString()}", #"[^\d\s\.:]", string.Empty);

If RDBMS type is DateTime then why should we convert to string and then parse it back to DateTime? Let's do it direct:
DateTime datetime = Convert.ToDateTime(row["datetime"]);
and let .net convert boxed DateTime (row["datetime"] is of type object?) to DateTime

There are a couple issues--at the highest level, your ParseExact method is encountering a Date Time string that does not match the supplied format.
According to the code you posted, the expected format of is dd.MM.yyyy HH:mm:ss, and in your exception exception, shows a Date time string (8/2/2021 2:00:00 PM) that does not match:
contains / and your expected format has .
dd is a 2-digit day, but the input date time string only has single digit days
MM expects a two digit month and the input date time only has a single digit month
the string contains AM/PM, and your format neglects to account for that.
Finally it's not clear if your date format is Month Day Year, or Day Month year.
The second issue, is that ParseExact should be enclosed in a try/catch block, so that your code can handle the case when an unexpected formatted date time string is passed in, and not crash.
To solve this, wrap your call into a try/catch, and gracefully handle the FormatException
And then make sure the Format string matches the expected input string.
Here is the .NET reference for the various DateTime format tokens

The error message is letting you know the issue.
You have :
DateTime datetime = DateTime.ParseExact(row["datetime"].ToString(), "dd.MM.yyyy HH:mm:ss", CultureInfo.InvariantCulture);
Notable, you are saying that the date format is going to be "dd.MM.yyyy HH:mm:ss"
Then your error message is saying that you couldn't parse :
8/2/2021 2:00:00 PM
Which is essentially a format of "d/M/yyyy h:mm:ss tt" (Assuming that days come before months).
If you change your code to :
DateTime datetime = DateTime.ParseExact(row["datetime"].ToString(), "d/M/yyyy h:mm:ss tt", CultureInfo.InvariantCulture);
You should be good to go. DateTime.ParseExact does what it says on the tin, it parses the date format exactly how you say it should come. If you aren't sure you can use DateTime.Parse() (But you can occassionally run into issues where days/months are around the wrong way).
Tested using the following code :
var myDateString = "8/2/2021 2:00:00 PM";
DateTime datetime = DateTime.ParseExact(myDateString, "d/M/yyyy h:mm:ss tt", CultureInfo.InvariantCulture);
Console.WriteLine(datetime.ToString());

DateTime.TryParseExact() not recognizing the PM designator, AM working as expected

I am writing a program to read log files, converting timestamps along the way. Currently, I am using DateTime.TryParseExact() to quickly analyze timestamps, ensuring things are correct. The issue I am running into is only AM designators are being recognized, PM are working without issue. I have isolated the issue in the below snippet:
string format = "M/dd/yyyy H:mm:ss tt";
string teststringPM = "1/21/2019 3:25:32 PM";
string teststringAM = "1/21/2019 3:25:32 AM";
DateTime placeholderPM;
DateTime placeholderAM;
DateTime.TryParseExact(teststringPM, format, System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None, out placeholderPM);
DateTime.TryParseExact(teststringAM, format, System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None, out placeholderAM);
Console.WriteLine("placeholderPM:");
Console.WriteLine(placeholderPM.ToString());
Console.WriteLine("placeholderAM:");
Console.WriteLine(placeholderAM.ToString());
The output from this looks like:
placeholderPM:
1/1/0001 12:00:00 AM
placeholderAM:
1/21/2019 3:25:32 AM
We can see that the placeholderPM is the default new datetime value. I have tried changing the IFormatProvider to en-US, without any change in behavior.
Any insight greatly appreciated!

It looks like you might be using the 'H' identifier instead of 'h'. This is an expected behavior as a upper case 'H' is used for 24 hour time. Using a lower case 'h' should resolve this issue.
For example, the format would become:
string format = "M/dd/yyyy h:mm:ss tt";
This goes into more detail

DateTime : parsing unknown format

The following code will output 2018-10-03 16:40:50 (instead of 2018-03-10) :
DateTime dateTime = new DateTime();
DateTime.TryParse("10/03/2018 4:40:50 PM", out dateTime );
MessageBox.Show(dateTime.ToString());
The following code will parse the date correctly (2018-03-10)
dateTime = DateTime.ParseExact("10/03/2018 4:40:50 PM", "dd/MM/yyyy h:mm:ss tt", System.Globalization.CultureInfo.InvariantCulture);
MessageBox.Show(dateTime.ToString());
Is there any way to correctly parse this date without knowing the exact format?
The current culture of the server application is en-ca (English Canada) and I don't know the exact format of the string date to parse. at least is there a way to parse the date without the curious swap between the month and day? Sounds like an easy question but lost a lot of time reading and tried almost anything found here.
Thanks

End up using TryParseExact with an array list of known format. (ugly but it work)
There is no way to detect MM-DD-YYYY versus DD-MM-YYYY. Known limitation (this format is finally not supported by our application)
Thanks!

DateTime parsing not working in Windows 7: The string was not recognized as a valid DateTime

Before i begin to explain ,i need to tell I have tried all the possible solutions for this problem that are provided in stackoverflow. But doesn't work in windows 7.
On windows 7 parsing datetime is not working.
i have tried the following code snippets
DateTime.ParseExact(arr[TransactionDateIndex], "M/dd/yyyy h:mm:ss tt", null);
DateTime.ParseExact(arr[TransactionDateIndex], "M/d/yyyy h:mm:ss tt",CultureInfo.InvariantCulture);
DateTime.Parse(DateTime.Parse(arr[TransactionDateIndex]).ToString("M/d/yyyy h:mm:ss tt"),CultureInfo.InvariantCulture);
I have an input file which has a transaction date column and it can be in any valid Date-Time Format, right now in the file its (MM/dd/YYYY) which i need to convert to "M/d/yyyy h:mm:ss tt" format.
When running in XP this code works fine but in windows 7 even after trying ParseExact its showing the error.
Even if i use
if (DateTime.TryParse(input, out dateTime))
{
}
While running in windows 7,few records will be treated as invalid but same things will be parsed in XP.

If you r current datetime format is MM/dd/yyyy
then you should use as
string dateString = "02/15/2015"
DateTime dateValue = DateTime.ParseExact(dateString , "MM/dd/yyyy", CultureInfo.InvariantCulture);
And if you want to convert that to some other format like you mentioned M/d/yyyy h:mm:ss tt then you can add the .Tostring('M/d/yyyy h:mm:ss tt') to the date.
As read trough your question one more time you mentioned that it can be in any valid Date-Time Format if that is so and if your cultureInfo is fixed.
You can use something like the following, but be aware that more than one format might be able to parse the same date. For example 10/11/12 can be parsed as yyyy/MM/dd or MM/dd/yyyy, which are both valid US date formats. MM/dd/yyyy is more common, so it appears first in the list and is the one returned by the code below (if you use it with a US culture instead of the culture in the example).
string testValue = "02/15/2015";
DateTime result;
CultureInfo ci = CultureInfo.GetCultureInfo("sl-SI");
string[] fmts = ci.DateTimeFormat.GetAllDateTimePatterns();
Console.WriteLine(String.Join("\r\n", fmts));
if (DateTime.TryParseExact(testValue, fmts, ci, DateTimeStyles.AssumeLocal, out result)) {
Console.WriteLine(result.ToLongDateString());
}
And if the CultureInfo is also not fixed then you should fetch all the available countercultures and then get all formats from them and then parse the date time .

It may be the case that there is an invalid date in
arr[TransactionDateIndex]
So try with a hard-coded date first. then you could check all the values passed to date.parse using debug.write() (windows app) or trance.write() (asp.net) by using debug class in System.Diagnostics and check the output window for any inconsistent date format.
Finally you could place this code at the top of the method where this code appears:
System.Threading.Thread.CurrentCulture = CultureInfo.InvariantCulture;

Convert "M/d/yyyy h:mm:ss tt" to "YYYY-MM-DDThh:mm:ss.SSSZ"

As title suggests, what I want to do is to convert my date-string e.g.
"6/6/2014 12:24:30 PM" (M/d/yyyy h:mm:ss tt) format
to
"YYYY-MM-DDThh:mm:ss.SSSZ" format.
I am trying in the following way. It's not giving me any exception, but I am getting the value like :
"YYYY-06-DDT12:24:30.SSSZ"
How can I exactly achieve this?
string LastSyncDateTime = "6/6/2014 12:24:30 PM";
DateTime dt = DateTime.ParseExact(LastSyncDateTime, "M/d/yyyy h:mm:ss tt",CultureInfo.InvariantCulture);
string result = dt.ToString("YYYY-MM-DDThh:mm:ss.SSSZ");

Data time Formats Can't recognize, the non Case sensitive chars in some systems due to their internal settings. Please refer the below code, which works fine
string result = dt.ToString("yyyy-MM-ddThh:mm:ss.SSSZ");
The above code will result as 2014-06-06T12:24:30.SSSZ
EDIT :
The below snippet will give you milliseconds as well
dt.ToString("yyyy-MM-ddThh:mm:ss.fffZ");

The simplest way is to use the "o" date formatter, like this:
dt.ToString("o");
This method will give you a timestring in the ISO 8601 format, something like this:
2014-12-25T11:56:54.9571247Z
but, since that ISO 8601 uses more than 3 decimal digits to define the second, if you only want to stop at milliseconds you can use the full formatting string and write down ss.fffzzz at the end, like this:
dt.ToString("yyyy-MM-ddThh:mm:ss.fffzzz");
and the result will be:
2014-12-25T11:56:54.957Z
For more informations you can refer to THIS LIST of date formatting options.

but I am getting the value like "YYYY-06-DDT12:24:30.SSSZ"
Let me explain why you get this result. Since there is no custom date and time format as YYYY, DD, SSS or Z, these characters are copied to the result string unchanged.
I strongly suspect you want to use yyyy, dd, fff and z format specifiers instead.
You can use DateTime.TryParseExact method like;
string s = "6/6/2014 12:24:30 PM";
DateTime dt;
if(DateTime.TryParseExact(s, "M/d/yyyy hh:mm:ss tt",
CultureInfo.InvariantCulture,
DateTimeStyles.None, out dt))
{
Console.WriteLine(dt.ToString("yyyy-MM-ddThh:mm:ss.fffz"));
}
Here a demonstration on Ideone.
Result will be 2014-06-06T12:24:30.000+3 on my machine because my time zone is UTC +3 right now. Time zone information might change based on your UTC value as well.

Try this -
dt.ToString("o");
dt.ToString("O");
you can see this link
hope this helps

You can use date in this format :("yyyy-MM-ddThh:mm:ss.000Z")
Or in manually by changing the time zone you can easily use the date set time zone to (GMT:000(Greenwich mean time)) in your setting tab and make sure data type is date/time.

Use yyyy for Year and dd for date
string result = dt.ToString("yyyy-MM-ddThh:mm:ss.SSSZ");

Here is the quick solution:
DateTime.UtcNow.ToString("yyyy-MM-ddThh:mm:ss.fffZ")