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Unable to convert DateTime from SQL Server to C# DateTime

I wrote a program in C# that uses dates. It takes the value from a SQL Server table. I was using Windows 7 and the program worked fine. I had this line of code:
DateTime fechaOperacion = Convert.ToDateTime(reader["FechaOperacion"]);
The reader returned a date in a 24h format and I was able to convert that to a DateTime variable.
Now I did a system upgrade to Windows 10 and that same line of code is throwing the following error:
String was not recognized as a valid DateTime. there is an unknown word starting at index 20.
And now the reader returns a.m. / p.m. format, and at index 20 there is a.m or p.m.
I have tried the following things:
Rephrasing the line of code to:
Convert.ToDateTime(reader["FechaOperacion"], System.Globalization.CultureInfo.InvariantCulture)
reader.GetDateTime(reader.GetOrdinal("FechaOperacion"));
Convert the culture to 24h format
System.Threading.Thread.CurrentThread.CurrentCulture = new CultureInfo(1033);
But none of that seems to work, I don't know what else to do.

Ultimately, the underlying problem here is storing a value that represents a date/time as textual data (meaning, some kind of [n][var]char({max|n}), or at a push: [n]text). This has multiple problems:
it takes more space
it cannot be sorted correctly / efficiently
it cannot be indexed correctly / efficiently
it cannot be filtered correctly / efficiently
it leads to parsing errors between client and server
it has all sorts of localization and internationalization problems
What you should have is a datetime / date / time / etc column. This is then stored as a number (not a string) that requires zero parsing and will work reliably without any conversion problems.
Note: it could be that you are storing it correctly but formatting it inside the select statement of your query. In which case, just don't do that; return the date-time raw, and let the receiving client worry about how to display it.

Depending on your actual format, you can define a suitable format list and do a conversion like below
string[] mfs = { "MM/dd/yyyy HH:mm:ss", "MM/dd/yyyy h:mm:ss tt"};
var dat = DateTime.ParseExact("04/23/1945 8:45:22 PM", mfs,
System.Globalization.CultureInfo.InvariantCulture,
System.Globalization.DateTimeStyles.None);

How to remove time from iTextsharp PDF when i made it by getting the data from Datagridview in C#?

I am making a program windows application by C# and MySQL, which uses itextsharp in the reports, but when I run the report the date field contain time also same as in the picture below, in the database the field is Date only, and I make the formats of datetimepicker and datagridview as "short" ... How can I solve this problem?

This is not really an iTextsharp problem. If you want to format precisely the date you may use:
DateTime date = DateTime.Now;
string dateString = date.ToString("dd/MM/yyyy");
There are other options but you see the pattern.

Parse date when format is not constant c#

normally i parse date like this way
DateTime.Parse()
DateTime.ParseExact()
i am in situation where user run exe and pass date as argument. so user can give date with various format like
dd/MM/yyyy
MM/dd/yyyy
dd-MM-yyyy
MM-dd-yyyy
yyyyMMdd
so i have to parse that date. when date format is yyyyMMdd then i am parisng date like this way DateTime.ParseExact(this.enddate, "yyyyMMdd", CultureInfo.InvariantCulture).ToString("yyyy-MM-dd");
so guide me what code i should write to parse date which work for any date format. thanks

I would recommend that you standardize on a single format. Otherwise you will run into ambiguous dates in cases where you have dates that can be parsed by different formats, but represent different dates in both
Ex:
dd-MM-yyyy
MM-dd-yyyy

what code i should write to parse date which work for any date format
As a technical answer, you can pass multiple formats to DateTime.TryParseExact() via a string array containing all acceptable formats.
Practically, though, the others have already pointed out that there is no way to tell the difference between months and days when the format isn't strictly enforced.
One possible solution is to have the user pass the date in as three separate arguments, each flagged with some kind of indicator such as /y2013 /m11 /d12 or maybe y:2013 m:11 d:12. You can even mash them together like /y2013/m11/d12. Then you can use Regular Expressions to parse out the parts, or even just plain old string manipulation.

There's no built in way to parse dates which work for ANY format. However, you can quite easily define your own format using DateTimeFormatInfo, letting you convert more or less any format to a proper date, as long as you know the format ahead of time.

In every major website you enter the date using comboboxes for day/month/year
or some datetime widget. So I don't see a reason to use a textbox. If you really
need to, add a tooltip or a watermark with the predefined format and force the
user to it.

Changing DATETIME format in datatable column without using loop in C#

I have a Data Table that has a column 'subscribeDate' that has dates in MM/dd/yyyy hh:mm:ss format. I would like to change all the dates in this column to MM-dd-yyyy hh:mm:ss format. Is there a way that I can do it without running a loop?

I would hope that the values in the DataTable are of type DateTime or DateTimeOffset.
Assuming that's the case, they don't have a format. They're just dates and times. How you convert them to a string is up to you.
If they're not already in an appropriate data type, you should try to change how you obtain the data to start with. Convert it from a string format to a more appropriate data type as early as you possibly can, and only convert it back to a string when you really need to.
You haven't explained how you're displaying the DataTable, but most ways that I'm aware of allow you to specify a format string - that's where you would specify your MM-dd-yyyy HH:mm:ss format. (Note HH rather than hh, to get 24 hours.)

You should be able to do that, if there is not some culture restrictions in your app (I don't know how works your application) with convert method.
Somethign like this:
myTable.Columns["Date"].Convert(
val => DateTime.Parse(val.ToString()).ToString("dd/MMM/yyyy"));
Where convert function you can find in Is it possible to format a date column of a datatable?

Formatting DateTime in Excel XML worksheet

When generating an Excel XML workbook with ASP.NET i'm appending 3 cells in a row like:
sb.Append(String.Format(
"<Cell><Data ss:Type=\"DateTime\">{1}</Data></Cell>{0}",
Environment.NewLine,
item.DateAvailable.ToString("yyyy-MM-ddTHH:mm:ss.fffffffK")
)); // newlines added for readability;
item.DateAvailable is a C# DateTime that has to end in Excel as An cell-DateTime.
so
value from debug in VS2010 in asssembly:
2011-08-15T01:16:06.8470000
value in excel:
40761,2299142361
after manual conversion of the cell formatting:
6-08-11 5:31
However, when converting it outputs in excel as a number, and the cell formatting is set to custom. When I manually change the cell formatting back to DateTime the correct??? date is shown in the correct format. But of course, the business would not accept such a raw data they have to convert themselves ;)
Perhaps I am using the wrong StringFormat?
What is the correct format to write a DateTime in this case and output it in Excel in the correct DateTime Format?

Yes, you are using a "wrong" string format, i.e. a format that Excel does not automatically recognize (or parse appropriately) as a date. Drop the T. Also, the date-time format in Excel rounds at least to milliseconds, if not to integer seconds, so you might as well drop some of those decimal places.
This should work:
item.DateAvailable.ToString("yyyy-MM-dd HH:mm:ss.ffff")