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How does DateTime.TryParse know the date format?

Here is a scenario.
You have a string that represents a date i.e. "Jan 25 2016 10:10 AM".
You want to know whether it represents a date in a specific culture.
You want to know what dateTime pattern satisfies this date string.
Example:
Date string is "Jan 25 2016 10:10 AM"
Culture is en-US
The POSSIBLE format for it could be "MMM dd yyyy HH:mm tt"
Implementation:
To get the list of all dateTime patterns you can get a CultureInfo.DateTimeFormat.GetAllDateTimePatterns()
Then try the overloaded version of DateTime.TryParseExact(dateString, pattern, culture, DateTimeStyles.None, out resultingDate) for each of the patterns above and see whether it can parse a date.
That should give you the needed dateTime pattern.
HOWEVER if we iterate all those patterns it will not find any matches!
This is even more weird if you try and use a DateTime.TryParse(dateString, culture, DateTimeStyles.None, out resultingDate) and it DOES parse the correct date!
So the question is how come the DateTime.TryParse knows the pattern of a date string when this info is not a part of CultureInfo and how to get to this info in a culture?
Thanks!

I agree with xanatos, there is no perfect solution for that and you can't assume that every format GetAllDateTimePatterns returns can be perfectly parsable with Parse or TryParse methods.
From DateTimeFormatInfo.GetAllDateTimePatterns;
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.
If you see Remarks section on the page, there are only 42 formats that can be parsed by TryParse method in 96 formats that GetAllDateTimePatterns method returns for it-IT culture for example.7
Tarek Mahmoud Sayed responded as;
Parse/TryParse are implemented as finite state machine so it doesn’t
really use the date patterns in parsing. It just split the parsed
string into tokens and try to find if the token match specific part of
the date (like Month, day, day of week…etc.). in the other hand
ParseExact/TryParseExact will just parse the string according to the
passed format pattern.

In short, Parsing is really hard because there are a lot of things that can trip it up. And someone in some government could suddenly decide that country X should use D/M/Y instead of M/D/Y, or could have someone entering data used to the other format.
I talk a little about this on a blog post (toward the bottom-ish) https://web.archive.org/web/20190110065542/https://blogs.msdn.microsoft.com/shawnste/2005/04/05/culture-data-shouldnt-be-considered-stable-except-for-invariant/
DateTime.Parse attempts to guess what the input might be based on the pattern(s) and separators it sees in the specified culture. Unfortunately, some cultures are REALLY hard to guess at. For example, . has been used for time formats in some locales, so is 1.1.1 12.12.12 the 12th day of December 2012? Or the 1st day of January 2001?
ParseExact (as the other answers suggest) is more reliable as you can tell it exactly what you're looking for - even better, you can also tell the user exactly what to enter. (Hopefully this is human input). Unfortunately it requires the user to follow the template.
This is also why most date controls you encounter, especially on the web, have separate fields for month, day & year.
For machine readable formats its best to spit it out in some standard format and read it back in with that exact same format. We've had customers send data from one country to another using the CurrentCulture and wonder why their vendor can't read it ;-)

convert system date to M/d/yyyy format irrespective of system format using C#

How can I get today's date in M/d/yyyy format irrespective of system format of date using C#?
DateTime.Now.Tostring('M/d/yyyy')
is working only if the system date is in format dd/MM/yyyy or M/dd/yyyy but is not working in case yyyy-MM-dd format.
Eg:
if system date is 2013-06-26 then DateTime.Now.Tostring('M/d/yyyy') is converting the date into 06-26-2013 but not in 06/26/2013

Use CultureInfo.InvariantCulture to enforce / as date separator:
DateTime.Now.ToString("M/d/yyyy", CultureInfo.InvariantCulture)
Ideone

Looks like you just need to use CultureInfo.InvariantCulture as a second parameter in your .ToString method.
Console.WriteLine(DateTime.Now.ToString("M/d/yyyy", CultureInfo.InvariantCulture));

Using the invariant culture is the correct solution if you always want the same DateTime to produce the exact same string on multiple systems (or even on the same system due to culture changes). However, be aware that if this is user-visible, you're giving up the possibility of internationalization (for instance, if you display day or month names, they will be in English regardless of what language the user speaks). To only ensure that slashes are not replaced with another date separator, use single quotation marks:
DateTime.Now.Tostring("M'/'d'/'yyyy");
Edit:
Also, if your users are using different date formats, there's a good chance they're also using different time zones. If this DateTime needs to make sense across multiple systems, consider using DateTime.UtcNow. This will also protect you against potential bugs due to a user changing their time zone (when travelling, say) or daylight saving/summer time beginning/ending. If you're just displaying the string to the user at the current instance and not persisting it, DateTime.Now is probably what you want. In that case, however, I'd question why you're trying to mess with the format they've chosen.

Like this:
DateTime.Now.Tostring("M'/'d'/'yyyy");
The apostrofe forces the ToString() method to use the delimiter that you specified.
However, I would let the user choose a culture and use that cultures default formatting instead.

Format a date using the current thread culture Vs double digits for month and day?

I want to display some rows of data on a web page where one column is a DateTime.
I want the date format to be displayed based on the current thread culture.
Right now, I'm doing this (dt is a DateTime):
string s = dt.ToString(Thread.CurrentThread.CurrentCulture.DateTimeFormat);
It's working well, however, on some culture, months and days are represented as only one digit (hours too), for example:
8/8/2011 8:57:59 AM
I would like the date to be displayed like this:
08/08/2011 08:57:59 AM
It would be easier to read (and prettier) when there's a list of rows.
I saw that there's a String.format method I could use, but that makes the current culture irrelevant.
Is there a way to achieve what I'm trying to do?

The solution provided here might be useful.
I see only a single solution - you should obtain the current culture display format, patch it so that it meets your requirement and finally format your DateTime value using the patched format string.

Make a custom culture.
Base it on the current thread culture.
Modify the settings you want to override.
Then either set it back into the thread as the culture or use it temporarily during the format operation.
We currently do this to format all dates in an internationally unambiguous form ddMMMyyyy where MMM is only English three-letter abbreviations, yet obey local numeric formatting rules ./, etc.
The relevant properties to override would be here.

If you want to show it based on the current culture, then what is the problem? If you want a specific format, you have to specify that.
string text = myDateTime.ToString("{0:[your format]}");

I believe this defaults to the server format - but what if you try specifying "u" as the format code which will put the year first then I think two digits.

You can use
String.Format("{0:dd/MM/yyyy hh:MM PM ", yourDatetime)
The date separator / (slash) and time sepatator : (colon) will be rewritten to characters defined in the current DateTimeForma­tInfo.DateSepa­rator and DateTimeForma­tInfo.TimeSepa­rator.
EDIT: Forgot to add object param needed to the string.format

using System.Globalization;
private static CultureInfo defaultCulture = new CultureInfo("nl-NL");
public static CultureInfo GetCurrentCulture()
{
List<CultureInfo> badCultures = new List<CultureInfo>();
badCultures.Add(new CultureInfo("en-US"));
if (badCultures.Contains(System.Threading.Thread.CurrentThread.CurrentCulture))
return defaultCulture;
return System.Threading.Thread.CurrentThread.CurrentCulture;
}

C# Regex, formatting a time

I'm making a timecard program, where the user enters the start time, end time, and project name. They can click "Add" and it adds the entered details to a listbox.
I don't allow the data to be added unless the start time and end time are formatted like this: 08:14am It gets annoying, though, having to enter it exactly like that each time, so I decided to, via regexes, have it automatically format. So if you enter 8:14, it will change it to 08:14am. How can I accomplish this via Regex.Replace()?
If you have any other methods, don't hesitate to list them.
Edit 1: I also have other replacements in mind; for example 814 goes to 08:14am, and 8 goes to 08:00am
Edit 2: So Now I'm using this code:
string[] formats = { "h:mm", "h", "hh", "hmm", "hhmm", "h:mmtt", "htt", "hhtt" };
DateTime dt = DateTime.ParseExact(t.Text, formats, new CultureInfo("en-US"), DateTimeStyles.None);
t.Text = dt.ToString("hh:mmtt").ToLower();
And it replaces some things, like h:mm but not others like h.

Have you looked at DateTime.Parse? It accepts "08:14am", "8:14"[1] and other variants, and returns a DateTime set to 8:14 am on today's date.
[1] In the UK culture at least -- consider providing a CultureInfo parameter depending on whether you want to pay attention to the user's local format preferences, or adopt a fixed format.
EDIT I also have other replacements in mind; for example 814 goes to 08:14am, and 8 goes to 08:00am
Take a look at DateTime.ParseExact: you can provide an array of valid time formats (such as "hh:mm", "hmm", "h" etc.)

Coming at it from a completely different direction, how about using a mask on the input?
For example, there are a number of JavaScript and jQuery tools available to do this.
Using this approach retains some user control over the input, and lets the user see their input.

have you considered using a DateTimePicker? Making the user write the time looks like more of a hassle.
See e.g. http://msdn.microsoft.com/en-us/library/system.windows.forms.datetimepicker.aspx

I think regex are overkill for this. Why not simply append the "am" at the end, in case nto already added is a 2 line code in case you are working with strings in the method.

I would recommend against using regular expressions if possible (as others have said, it seems like overkill) and try using datetime formats. See here: http://msdn.microsoft.com/en-us/library/8kb3ddd4.aspx
The "HH" format tag will format the hour as a 24-hour time from 00 to 23. Traditionally, 12 hour times are not zero-padded to the left, but 24 hour times are (at least as often as I see them), probably to avoid confusion between 1800 and 0800 . Alternatively, if you don't want 24 hour times, you could probably just left-pad the hour part of the string with a zero to up to 2 digits.
EDIT:
Based on your new requirements, I'd say write a simple parser to let users input "814" meaning "08:14 AM" and make use of the time-formatting functionality for display.

CultureInfo & DateTimeInfo: How to check if is 24 hour time?

I'm modifying a globalized web application which uses stored CultureInfo for each logged in user.
The client would like time data entry to be localized. Displaying is not a problem as the formatting is already available. However I need to detect if the current cultureinfo is for 24 hour time or am/pm so I can display the correct input boxes (not just a textfield).
My initial idea was to check the DateTimeInfo property of CultureInfo and see if the ShortTimePattern contained a capital H or a lower case h but this didn't feel robust enough for me.
Is there a better way? I've read the class properties of both but unless I'm missing something, I can't see any existing methods or properties.

I don't think there is a better way to obtain that information. The time pattern for a culture could contain anything (a user could even create a custom culture where the ShortTimePattern is "\hello" and then DateTime.ToString() would return "hello" for any time). In that case how could the framework determine if that CultureInfo is in 24-hour or 12-hour format?
So a "normal" DateTimeFormatInfo.ShortTimePattern will necessarily contain either a 'h' or a 'H', otherwise the hour will not be displayed. I think you can follow your initial idea and check for that. You can also check that the 'h' or 'H' is not escaped with a \ like in my "\hello" example because that would not represent the hour :)

Checking for 'H'/'h' seems more robust than checking for the AM/PM Designator.
A good example is en-gb:
The time format string is HH:mm and the AM/PM designators are set to AM/PM
Windows will display the time in 24h format!
This seems to be an inconsistent definition but checking for 'H' fixed my bug.

The most robust way is to check if DateTimeFormatInfo.AMDesignator is an empty string.
if (DateTimeFormatInfo.CurrentInfo.AMDesignator == "")
//24hour format
else
//12hour format