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convert string in unknown format to date in c#

I have searched stackoverflow for an answer but no luck. I am developing a windows application and I have some strings in different date formats,
eg.
dd/MM/yyyy
MM/dd/yyyy
MM-dd-yyyy
dd-MM-yyyy
dd/MM/yyyy hh:mm::ss
MM/dd/yyyy hh:mm::ss
etc...
But I need to convert in to a common format - dd/MM/yyyy. The application can run in any windows machines in different culture.
What is the correct way to do it?
EDIT: One more thing I may not know what the format of incoming string.
Thanks in advance.

Use DateTime.ParseExact with the different patterns as formats.
If after parsing you really need to use a string representation, use the ToString method of the DateTime with the explicit format that you're interested in (so that it is culture-invariant). It's better however to keep the DateTime because this is format-agnostic.

You could distinguish between those formats that use different separators (i.e. "/" vs "-"). But how would you know if date such as 10/11/2010 represents 10th of November or 11th of October? If one number is not bigger than 12, there is no reliable way to do this without knowing an exact format.
As others have pointed out, if you do know the exact format, then you can use DateTime.ParseExact.

If you are processing some import file with a lot of dates in the same unknown format, you could try different formats and hope there is exactly one that doesn't give format errors.
Or to put it another way: split the "dates" into three numbers and check the range of values for each of those numbers. Values > 1900 will be years. If you find values from 1 to 31, those will be days.
Values from 1 to 12 might be months, but could also be days. Try and identify each of the parts.
The best way is to ask the supplier of those dates for the format.

To run this program on different culture, i think you should creat a function to indentify the culture of this string format and then use Datetime.Parse

datetime.TryParseExact with different formats of values

I have a json string that contains the values for a datetime and a parsing mechanism that looks like this:
if (DateTime.TryParseExact(TheUserTimeString, "M.d.yyyy.HH.mm", CultureInfo.InvariantCulture, DateTimeStyles.None, out TheUserTime))
{
TheObject.UserDateTime = TheUserTime;
}
The string TheUserTimeString is generated on the client. It can be 12.20.2011.13.21 and the code works fine but when it's 12.20.2011.13.2 the code breaks because the minutes are in one digit. And when the month is also in one digit... who knows.
What would be a better way to rewrite this parsing code so that the string gets parsed correctly every time.
Thanks for your suggestions.

Use the string "M.d.yyyy.HH.m", a single m denotes minutes without the leading 0. Source.

Your DateTime format string just needs to be: "M.d.yyyy.H.m".
This allows for months, days, hours and minutes to be expressed as single digit values.
See here for the MSDN page documenting the valid formats of this string for further information.

Date and time regular expression

I'm looking for a regular expression which matches the following datetime format:
dd-MMM-yyyy HH:mm:ss (15-Sep-2011 16:00:47)
Currently I only have the regex for date which looks something like this:
^(3[0-1]|2[0-9]|1[0-9]|0[1-9])[\s{1}|\/|-](Jan|JAN|Feb|FEB|Mar|MAR|Apr|APR|May|MAY|Jun|JUN|Jul|JUL|Aug|AUG|Sep|SEP|Oct|OCT|Nov|NOV|Dec|DEC)[\s{1}|\/|-]\d{4}$
Any ideas for the time part?
It's ok guys I found the solution. Submitting for anyone who wants to utilise it.
(3[0-1]|2[0-9]|1[0-9]|0[1-9])[\s{1}|\/|-](Jan|JAN|Feb|FEB|Mar|MAR|Apr|APR|May|MAY|Jun|JUN|Jul|JUL|Aug|AUG|Sep|SEP|Oct|OCT|Nov|NOV|Dec|DEC)[\s{1}|\/|-]\d{4}\s(20|21|22|23|[0-1]?\d):[0-5]?\d:[0-5]?\d

Must it be a regex?
DateTime.TryParseExact will work much better.
DateTime myDate;
// dd-MMM-yyyy HH:mm:ss (15-Sep-2011 16:00:47)
if (DateTime.TryParseExact(dateAsString,
"dd-MMM-yyyy HH:mm:ss",
new CultureInfo("en-US"),
DateTimeStyles.None,
out myDate))
{ ... }

Err, it appears to me that the date bit is the hard one :-) But you can use the same methods for the time.
Assuming you have two fixed digits, you can use something like:
(([01][0-9])|(2[0-3])):[0-5][0-9]:[0-5][0-9]
The first bit is the slightly tricky bit since you want one of:
0 or 1 followed by a digit.
2 followed by 0 thru 3.
The minutes and seconds are both:
0 thru 5 followed by 0 thru 9.
If you want to allow single-digit hours, just replace [01] with [01]?.
You may also want to consider the possibility that people may enter nov or OcT as the month, rendering your regex less useful.
This could be solved with a case-insensitive version which would also reduce the size of the regex as well, requiring only one string per month.

As Joel suggested above that I should add my solution as an answer here.
So here it is:
(3[0-1]|2[0-9]|1[0-9]|0[1-9])[\s{1}|\/|-](Jan|JAN|Feb|FEB|Mar|MAR|Apr|APR|May|MAY|Jun|JUN|Jul|JUL|Aug|AUG|Sep|SEP|Oct|OCT|Nov|NOV|Dec|DEC)[\s{1}|\/|-]\d{4}\s(20|21|22|23|[0-1]?\d):[0-5]?\d:[0-5]?\d
Do let me know if it works out.

Change date to Universal Time Format changes my date wrongly I can't get error

I am trying to simply change the date format from the datatable to universal time format but it formats it wrongly as if I have date for August 7 it changed it to August 8 after formatting it to universal date time. My code for formatting date is,
DateVar[runs] = DateTime.Parse(Convert.ToString(output.Tables[0].Rows[runs][0])).ToUniversalTime().ToString();
Don't get in to code its correct and its a part of loop so "run" is loop and output is data set having one table I have first data in table is "Sunday, August 07, 2011 10:52 PM" and it was converted to "8/8/2011 5:52:00 AM" after implementing universal time format.
Hopes for your suggestions

Universal time isn't a format - it's a time zone, effectively. It's not clear what you're trying to do, but converting a "local" DateTime to "universal" DateTime will usually change the time. If you don't want that to happen, don't call ToUniversalTime.
It's a pity that the .NET date/time API isn't as clear as it could be - the DateTime type itself has some horrible ambiguities about it. I'm trying to improve the situation with my Noda Time project, but you will need to understand what time zones are about etc.
Personally I would suggest not using simply DateTime.Parse or just calling ToString unless you're absolutely sure that the default format is what you want. I usually call DateTime.ParseExact and specify the expected format (and usually CultureInfo.InvariantCulture unless it's a user-entered string) - and likewise I provide a format string to the ToString call.
In your code you're simply converting a string to a string - what are you attempting to accomplish? If you're just trying to change the format (e.g. to dd/MM/yyyyTHH:mm:ss) then you don't need to call ToUniversalTime but you do need to provide the format string.
I suggest you split your code out into several statements to help you debug this (and for general code clarity):
Fetch the string from the DataTable, if you really need to (if it's already a DateTime, there's no point in converting it to a string and then back again)
Parse the string (again, assuming you need to)
Perform any conversions you need to
Format the DateTime with an explicit format string
Now if any single operation is causing a problem, you can isolate it more easily.

If I run ToUniversalTime() from Greenwich it will give same time but if i do it while I live some where else it will get an offset date time object of + or - hours depending on position.

Parsing dates without all values specified

I'm using free-form dates as part of a search syntax. I need to parse dates from strings, but only preserve the parts of the date that are actually specified. For instance, "november 1, 2010" is a specific date, but "november 2010" is the range of dates "november 1, 2010" to "november 30, 2010".
Unfortunately, DateTime.Parse and friends parse these dates to the same DateTime:
DateTime.Parse("November 1, 2010") // == {11/1/2010 12:00:00 AM}
DateTime.Parse("November, 2010") // == {11/1/2010 12:00:00 AM}
I need to know which parts of the DateTime were actually parsed and which were guessed by the parser. Essentially, I need DateTime.Parse("November, 2010") == {11/-1/2010 -1:-1:-1}; I can then see that the day portion is missing and calculate the range of dates covering the whole month.
(Internally, C# has the DateTimeParse and DateTimeResult classes that parse the date and preserve exactly the information I need, but by the time the date gets back to the public interfaces it's been stripped off. I'd rather avoid reflecting into these classes, unless that's really the only route.)
Is there some way to get DateTime.Parse to tell me which format it used to parse the date? Or can the returned DateTime have placeholders for unspecified parts? I'm also open to using another date parser, but I'd like it to be as reliable and locale-flexible as the internal one. Thanks in advance.
EDIT: I've also tried ParseExact, but enumerating all of the formats that Parse can handle seems nearly impossible. Parse actually accepts more formats than are returned by DateTimeFormatInfo.GetAllDateTimePatterns, which is about as canonical a source as I can find.

You could try using TryParseExact(), which will fail if the data string isn't in the exact format specified. Try a bunch of different combinations, and when one succeeds you know the format the date was in, and thus you know the parts of the date that weren't there and for which the parser filled in defaults. The downside is you have to anticipate how the user will want to enter dates, so you can expect exactly that.
You could also use a Regex to digest the date string yourself. Again, you'll need different regexes (or a REALLY complex single one), but it is certainly possible to pull the string apart this way as well; then you know what you actually have.

Parse parses a whole lot of stuff that no sane person would enter as a date, like "January / 2010 - 21 12: 00 :2". I think you'll have to write your own date parser if you want to know what exactly the user entered.
Personally I would do it like KeithS suggested: Parse the string with Parse and only call your own parse function if there's a 0 in one of the fields of the DateTime object. There are not that that possibilities you need to check for, because if the day is 0, the time will be 0, too. So start checking year, month, day, etc..
Or simply instruct the user to use specific formats you recognize.

Essentially, I need
DateTime.Parse("November, 2010") ==
{11/-1/2010 -1:-1:-1}; I can then see
that the day portion is missing and
calculate the range of dates covering
the whole month.
What you want is an illegal DateTime because you cannot have a negative hours/seconds/minute/day values. If you want to return something else other then a legal DateTime you have to write your own method which does NOT return a DateTime.
Is there some way to get
DateTime.Parse to tell me which format
it used to parse the date? Or can the
returned DateTime have placeholders
for unspecified parts? I'm also open
to using another date parser, but I'd
like it to be as reliable and
locale-flexible as the internal one.
Take a look here http://msdn.microsoft.com/en-us/library/w2sa9yss.aspx
You are going to have to manually keep track of what is entered to do this task. The only solution is to make sure the input is in the correct format.

I used this method that goes back to the original string in order to check for existence of the day and the year:
For days, the original string must contain a 1 as integer if the day was specified. So, split the string and look for a 1. The only exception occurs when the month is January (#1 month), so you should check for two 1s or a 1 and "January" or "Jan" in the original string.
For years, the original string must contain a number that can be a year (say, from 1900 to 2100). Other possibilities may be the use of an apostrophe, or things like 02-10-16, which you can recognize by the fact that there are exactly three numbers.
I know that this is pretty heuristic, but it's a fast and simple solution that works in most cases. I coded this algorithm in C# in the DateFinder.DayExists() and DateFinder.YearExists() methods in the sharp-datefinder library.