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Culture dependent NumberFormat for dates in C# VSTO Excel

I'm relatively new to C# and I'm still learning a lot. I also searched stackoverflow for similar problems but got to a point where I'm stuck.
In my VSTO Excel project I'd like to output dynamic data coming from a database with a predefined number format per column also coming from the database. The user can define his own layout of the report. Everything is working just fine, I have just problems formatting the date value.
When it's about to output a date column, the format should be automatically be determined by the program instead of predefining it in the database as well. The date value gets already inserted in the Excel sheet as the decimal number it should be. Everything okay here. When I manually format it in Excel itself, it is displayed correctly.
But I want to format the date with C# like it would be formatted when I do it manually by clicking on the short date format in Excel. And it should work in every country, regardless in which language the office package was installed.
I'm working with a German installation and I expect the format to be "TT.MM.JJJJ". When formatting the column with this fixed string, it works perfectly (at least in Germany I guess).
It makes no difference if formatting it with
CurrentWorksheet.Columns[queryColumn.Position].NumberFormat = "TT.MM.JJJJ";
or
CurrentWorksheet.Columns[queryColumn.Position].NumberFormatLocal = "TT.MM.JJJJ";
Both end up in the same (correct) result. That the first line is working as well is the first thing which was unexpected for me, but that's not the main point. I tried to format it dynamically by replacing "TT.MM.JJJJ" with the following lines:
// Leads to dd.MM.yyyy
System.Globalization.DateTimeFormatInfo.CurrentInfo.ShortDatePattern;
// Fixed German culture still leads to dd.MM.yyyy
CultureInfo.CreateSpecificCulture("de-DE").DateTimeFormat.ShortDatePattern;
// MM/dd/yyyy - also not helpful
CultureInfo.InvariantCulture.DateTimeFormat.ShortDatePattern;
When using the first or second ShortDatePattern the date 27.08.2014 is shown as "dd.08.yyyy". Excel simply doesn't unterstand "dd" or "yyyy". I thought at least when using NumberFormat instead of NumberFormatLocal it would work, because it's the global format, but it doesn't. It feels like Microsoft has implemented the date formatting more complicated that it could be.
How would I determine the correct date pattern here?

The answer (it still is a kind of workaround) is pretty much found in another question:
How to set localized short-date format for a cell in Excel with C#?
There's just a very small difference I had to make in the method for constructing the ShortDatePattern (the Application parameter for DateFormatComponentCodes).
public static string ConstructExcelShortDatePattern()
{
var systemDateComponentCodes = new DateFormatComponentCodes();
var excelDateComponentCodes = new DateFormatComponentCodes(Globals.ThisAddIn.Application);
string systemShortDatePattern = CultureInfo.CurrentCulture.DateTimeFormat.ShortDatePattern;
string excelShortDatePattern = systemShortDatePattern.Replace(systemDateComponentCodes.Year, excelDateComponentCodes.Year).Replace(systemDateComponentCodes.Month, excelDateComponentCodes.Month).Replace(systemDateComponentCodes.Day, excelDateComponentCodes.Day);
return excelShortDatePattern;
}
Thanks to Heinzi for finding a great answer and Jordan for providing such a useful class!

DateTime.Parse can format unusual format strings?

I was looking at a code in an application (Someone else wrote it),on some cases it worked fine and on some cases it gave exceptions,it was actually converting strings in datetime,here is the code
//5000 is the year,but what about "1" is it month or day ?,if its month
//then what about the day ?
DateTime time = DateTime.Parse("1.5000");//1.5000 doesn't looks a date to me ?
time.ToString();//returns "1/1/5000 12:00:00 AM"
//where as if I give this string to DateTime.Parse();
time = DateTime.Parse("2341.70");
//FormatException was unhandled
//String was not recognized as a valid DateTime.
A Confusing thought
How does this string "3.5000" (it matches the 1.5000 pattern) evaluates , does this means 3-3-5000 or 1-3-5000 ,the format is ambiguous its unclear and confusing !
My questions are,
What kind of formats can DateTime.Parse expects ?
Whats happening in the code above ?
Suggestions to improve the code ?

Many people have commented on the possible reasons for the parse that you have seen being successful but your question seems to have several separate parts...
1. What kind of formats can DateTime.Parse expects ?
DateTime.Parse has been written to be as inclusive as possible. Pretty much anything that it can find someway to make into a DateTime it will do its best to do so which means in addition to the usual familiar yyyy-MM-dd type formats more strange ones like M.yyyy or yyyy.M and so on.
2. Whats happening in the code above ?
That is very complicated because the DateTime.Parse method is itself very complicated. You can probably fidn the source code out there somewhere but the complexity made it very hard for me to follow. Without being able to give precise details I'm going to answer this the same as above. What is happening is that the framework is trying its best to give you a date back and not throw an exception. The date it gives is the best guess as to what you meant.
3. Suggestions to improve the code ?
It sounds like if you are getting parse exceptions that you are passing dates in formats that are unexpected. Without knowing what those inputs are its hard to say. Two things could improve your code though. Making sure a single consistent date format is used and then using DateTime.ParseExact to ensure that it conforms to the right format. You will remove all ambiguity this way but you will sacrifice flexibility.
The second option is to use DateTime.TryParse. This will attempt to parse your date and then return a boolean saying whether it succeeded or not. If successful the date parse will be returned in a ref parameter. This won't make your code any better at recognising unknown date formats but will let your code know when such an unparsable format crops up and you can deal with it (eg by providing user feedback reporting the wrong format and suggesting a correct one, or just by logging it or something else).
What the best method is depends mostly on where your input is coming from. If it is user input then I'd go with the second option. If it is automated input then you probably want to make sure your input is standardized and then use the first option. Of course circumstances always vary so this is not a hard and fast rule. :)

In regards to "2. Whats happening in the code above ?":
In some cultures, the date separator is a dot instead of a slash. So for example 13.12.2013 is a valid date (2013-12-13) in the format "dd.MM.yyyy". Now by whatever design choice, the day part in this example is not mandatory and if left out, is automatically filled with 1. So parsing 12.2013 would result in 2013-12-01. And therefore it's easy to see how 1.5000 would become 5000-01-01. 2341.70 can not be parsed, because 2341 is not a valid month. - So in this case 1.5000 is a "valid" date in the format M.yyyy.

Javascript equivalent to .NET's DateTime.Parse

I'm trying to build a validator that will work with .NET's DefaultModelBinder of using DateTime.Parse to convert a string from the form post to a DateTime. I don't want to have to wait until a date has been posted to the server for it to realize it was a bad date.
Currently jquery.validate uses the following code to validate date fields:
// http://docs.jquery.com/Plugins/Validation/Methods/date
date: function(value, element) {
return this.optional(element) || !/Invalid|NaN/.test(new Date(value));
}
However, due to Javascript's terrible Date parser, this:
275481/69/100089
Will evaluate as valid, to Sep. 12, 275760.
While on the other hand, this:
11-19-2013
Will evaluate as invalid.
Of course, I understand that C#'s DateTime.Parse() takes things like culture (localization) and leap year into account, and I could live with assuming a fixed (US) culture, and allowing "02-29-2013" on the client and kick it out at the server (ideally not, but it's acceptable).
But I can't believe someone hasn't put together a better date validator to work with C#'s DateTime.Parse() logic.
Maybe someone has, I just haven't found it -- which is why I'm posting here.
And I know I have several ways to go about this -- from incredibly simple (less accurate) to incredibly complex (more accurate), but I'm hoping someone has already gone down this road and found the sweet spot.

Datejs seems pretty robust to me. Its parse function supports over 150 cultures:
Date.parse("February 20th 1973")
And in case you need to parse a date string that is not valid in the current culture you can use the parseExact function:
// The Date of 15-Oct-2004
Date.parseExact("10/15/2004", ["M/d/yyyy", "MMMM d, yyyy"]);

In all honesty, your best bet is to perform an AJAX hit, and ask your ASP.net web-server to parse the string and return a Javascript date.
Javascript libraries easily get confused with different locales, e.g.:
GET /ParseDate.ashx?dateStaring=06/01/34 4:53:05 غ.و&locale=ar-SA
Which gets really complicated because:
"6/1/34" = November 19, 2012
The .NET framework, with Windows behind it, has support for a lot of different locales.

Instead of trying to find two Datetime implementations (one for JS and another for C#) that have similar validation and parsing, have you considered having the client 1)use its own library to validate the date and 2)parse and reformat the date to a C# friendly format?
This would allow you to use DateJS to get a very flexible front end for date inputs, make it easier to deal with the client side culture, and let your server side deal with a fixed format.

Have you tried passing your string into the constructor?
Here's a sample from https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Date
var birthday = new Date("December 17, 1995 03:24:00");

Regular Expression to detect yyyy-MM-dd

I use asp.net 4 and c#.
I need to use a WebControl of type Validation namely RegularExpressionValidator to detect data inputed in a TextBox that IS NOT in format yyyy-MM-dd (String).
Any idea how to write the RegEx to apply ot this control?
Thanks

Here's one possible regex:
^\d{4}-((0\d)|(1[012]))-(([012]\d)|3[01])$
Note: this will prevent months >12 and days >31, but won't check specific months for length (ie it won't block 30th Feb or 31st Apr). You could write a regex to do that, but it would be quite lengthy, and 29th Feb is always going to give you problems in regex.
I'd say if you need that kind of fine-grained validation, you're better off parsing the date with a date library; regex isn't the tool for you. This regex should be sufficient for basic pre-validation though.
I've also gone lenient on the year; just checking that it's four digits. But if you want some sort of sanity check (ie within certain bounds), it shouldn't be too hard to add. Foe example, if you want to match only dates in the this century, you would replace the \d{4} at the beginning of the regex with 20\d{2}. Again, trying to validate a date with excessive accuracy in regex is going to be difficult and you should use a date parser, but you can get basic century-level matching quite easily to prevent the user entering anything really silly.
Finally, I've put ^ and $ to tie off the ends of the string so it can't match if the user enters a valid date and extra characters as well. (You may want to add a string length validator for this as well).
Hope that helps.

Spudley's answer above allows 00 for day and month.
I fixed it :
^\d{4}-((0[1-9])|(1[012]))-((0[1-9]|[12]\d)|3[01])$
Note: neither of these expressions check for days in a month that are invalid, e.g. 04/31, 06/31 or 02/29 on non-leap years.

Regular expression \d\d\d\d-\d\d-\d\d should do the trick.

I would like to add a little change in Spudley's answer:
^\d{4}$|^\d{4}-((0?\d)|(1[012]))-(((0?|[12])\d)|3[01])$
so you can use date like 2013-5-5 (month and date is not necessary the zero but can be used)
Hope it helps.

Another implementation for ISO 8601 structured dates:
^\d{4}-\d{1,2}-\d{1,2}\s\d{1,2}:\d{1,2}:\d{1,2}.?\d{0,}$
It's not quite as strict, and will accept incorrect dates, but it should validate that it follows the ISO 8601 structure even if the date is a non-existent one. It should also be fairly simple to understand for anyone with a basic Regex understanding.
If you really want to ensure the date is correct, and work with it, run DateTime.TryParse() on it.

(19|20)[0-9]{2}-(0[1-9]|1[012])-(0[1-9]|[12][0-9]|3[01])
mach result:
1999-09-12

((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))-((0[13578])|(1[02]))-((0[1-9])|([12][0-9])|(3[01])))|((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))-((0[469])|11)-((0[1-9])|([12][0-9])|(30)))|(((000[48])|([0-9]0-9)|([0-9][1-9][02468][048])|([1-9][0-9][02468][048]))-02-((0[1-9])|([12][0-9])))|((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))-02-((0[1-9])|([1][0-9])|([2][0-8])))
This is the regex for yyyy-MM-dd format.
You can replace - with \/ for yyyy/MM/dd...
Tested working perfect..

Is there an API to convert from "YYYYMMDDHHMMSS.UUUUUU-TZO" format to C# DateTime?

Example: "20080807144334.410187-180" (-180 means GMT minus three hours. Rio de Janeiro in this case.)
That string format is returned when I query file creation/change/access times via WMI (that is not totally working; see here). I guess I could parse it the idiot way, extracting year, month etc. from the string positions. But I'd like not to reinvent the wheel. System.DateTime's constructors don't handle that format. Should I go on and do it the idiot way or is there something better?

You should be able to use DateTime.ParseExact or .TryParseExact to give it the specific format to use when parsing.
However, I don't think you can get it to read your time zone in that format (though I can't actually figure out how to get it to read a time zone in any format).
The rest of it would look like this:
DateTime.ParseExact("20080807144334.410187", "yyyyMMddHHmmss.ffffff", System.Globalization.CultureInfo.InvariantCulture)

You should take a look at the DateTime.TryParseExact method. It'll let you pass in your format that you're converting from.
http://msdn.microsoft.com/en-us/library/system.datetime.tryparseexact.aspx