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Parse year/month/day string with unknown dividers and optional time element into a DateTime, in C#

I have to parse date string in C#. The dates are all ensured to start with year, month, day. But I do not know what dividers will be between the 3 parts.
Additionally the date string may also include a time part after the date.
Basically as long as the format has the year first, month second, and day third, I should parse it as a valid date regardless of which dividers are used and whether a time is included. Any other date formats should be rejected as invalid.
I can not figure out how to do this without writing a long if/else.
How do I parse a string into a C# datetime object, given the restrictions mentioned?

You can check the length of the input string is at least 10 characters, and if it is, work out what the separator should be by looking at the 5th character in the string.
Then you can use the separator to construct a format string that you pass to DateTime.TryParseExact() to parse the date. You also have to truncate the date string to 10 characters to ignore any date part at the end.
An example implementation looks like this - it returns null if the date didn't parse; otherwise, it returns the correctly parsed date:
public static DateTime? ParseDateWithUnknownDivider(string dateStr)
{
if (dateStr.Length < 10)
return null;
char divider = dateStr[4];
if (DateTime.TryParseExact(
dateStr.Substring(0, 10),
$"yyyy\\{divider}MM\\{divider}dd",
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out DateTime result))
return result;
return null;
}
Note that this ignores the time part and will always return the time part as 00:00:00. If that's not what you meant, you will need to specify in your question what the time part would look like. For example, would it be separated from the date part by a space? And would it always be hh:mm:ss? And would it be 24hour clock?

try this code
string yourDateTimeString = ....;
string format="yyyy/MM/dd";
var dt = DateTime.ParseExact(yourDateTimeString,format,null,System.Globalization.DateTimeStyles.AssumeLocal);

C#: Code Optimization for Date Formatting

Using C#, I am trying to populate a variable with a known filename + date. The catch is that the date must always be the last day of the previous month. It must also have no slashes or hyphens, and the year must be two digits. For example, if it is now November 27 2015, I need my filename to be: Foobar_103115.txt
As a programmer who still has much to learn, I have written the clunky code below and it does achieve my desired result, even though it will obviously break after the end of this century. My code is written this way because I could not figure out a more direct syntax for getting the date I want, complete with the specified formatting.
My question is this: What would be a more elegant and efficient way of recreating the below code?
I have commented all the code for any novice programmers who might be interested in this. I know the experts I'm asking for help from don't need it.
public void Main()
{
String Filename
DateTime date = DateTime.Today;
var FirstDayOfThisMonth = DateTime.Today.AddDays(-(DateTime.Today.Day - 1)); //Gets the FIRST DAY of each month
var LastDayOfLastMonth = FirstDayOfThisMonth.AddDays(-1); //Subtracts one day from the first day of each month to give you the last day of the previous month
String outputDate = LastDayOfLastMonth.ToShortDateString(); //Reformats a long date string to a shorter one like 01/01/2015
var NewDate = outputDate.Replace("20", ""); //Gives me a two-digit year which will work until the end of the century
var NewDate2 = NewDate.Replace("/", ""); //Replaces the slashes with nothing so the format looks this way: 103115 (instead of 10/31/15)
Filename = "Foobar_" + NewDate2 + ".txt"; //concatenates my newly formatted date to the filename and assigns to the variable

Sounds like you want something more like:
// Warning: you should think about time zones...
DateTime today = DateTime.Today;
DateTime startOfMonth = new DateTime(today.Year, today.Month, 1);
DateTime endOfPreviousMonth = startOfMonth.AddDays(-1);
string filename = string.Format(CultureInfo.InvariantCulture,
"FooBar_{0:MMddyy}.txt", endOfPreviousMonth);
I definitely wouldn't use ToShortDateString here - you want a very specific format, so express it specifically. The results of ToShortDateString will vary based on the current thread's culture.
Also note how my code only evaluates DateTime.Today once - this is a good habit to get into, as otherwise if the clock "ticks" into the next day between two evaluations of DateTime.Today, your original code could give some pretty odd results.

c# Is it possible to subtract a day in the datetime format

I have the following simple example:
string dt = DateTime.Now.ToString("yyyy-MM-dd hh.mm.ss");
I can't change the DateTime.Now, but I can change datetime format yyyy-MM-dd hh.mm.ss. Following this example the result must be today's date, but I need to get yesterday's date with the same parameters except day (year, month, hours, minutes and seconds). E.g. 2015-08-23 12.09.59 must be 2015-08-22 12.09.59. So is it possible to use some "-" operator or something else inside the datetime format to achieve the result?

If you want yesterday's date, you can do this
string dt = DateTime.Now.AddDays(-1).ToString("yyyy-MM-dd hh.mm.ss");
DateTime.AddDays() lets you add number of days, positive for future date, negative for past date.

E.g. 2015-08-23 12.09.59 must be 2015-08-22 12.09.59. So is it
possible to use some "-" operator or something else inside the
datetime format to achieve the result?
No, it's not possible inside the DateTime format. you can not change any thing. Because it is only for define format of the Date to display in string format. Any addition or subtraction can only be done before converting it to string format as suggested by "Arghya C".
Can you explain your limitation so we can solve your problem.

If you can only influence the date time pattern, than use the roundtrip format and parse the returning string back to a date time, add the calculation and format it into the desired format:
var dateTimeString = badLibrary.GetDateTime("o");
var dateTime = DateTime.Parse(dateTimeString, null, DateTimeStyles.RoundtripKind);
var newDateTime = dateTime.AddDays(-1);
return newDateTime.ToString("yyyy-MM-dd hh.mm.ss");

Parsing times above 24 hours in C#

Suppose a time stamp (just time or date and time) where the time can roll over to the next day:
00:00:00 <- midnight
01:00:00 <- 1 AM
23:00:00 <- 11 PM
24:00:00 <- midnight, day + 1
25:00:00 <- 1 AM, day + 1
What would be a way to parse it easily into a C# DateTime that would perform the carry-over to the next day? In other words, "01:00:00" would become "0001-01-01 01:00:00" and "25:00:00" would become "0001-01-02 01:00:00".
EDIT:
I should mention that this fails miserably (i.e FormatException):
DateTime.ParseExact("0001-01-01 25:00:00", "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture);

Since you're trying to represent a period of time from an arbitrary point, rather than as a specific date, perhaps you would be better off using the System.TimeSpan class? This allows you to set values of more than 24 hours in the constructor, and can be used with DateTime objects like this:
System.TimeSpan timestamp = new System.TimeSpan(25, 0, 0);
System.DateTime parsedDateTime = new DateTime(0, 0, 0);
parsedDateTime = parsedDateTime.Add(timestamp);
Console.WriteLine(parsedDateTime.ToString("yyyy-MM-dd HH:mm:ss")); //Output as "0001-01-02 01:00:00"
NOTE: Code is untested.
EDIT: In terms of parsing the strings, I can't think of any basic .NET objects that parse strings with values greater than 23 for the hour (since 25 is an invalid hour of the day), but assuming that the format is consistent, you could create a very simple string parsing routine (or even a regular expression) to read the values individually, and load the constructor manually.

If you have an existing DateTime value you can add to, you can always use a TimeSpan:
string dt = "25:00:00";
int hours = int.Parse(dt.Split(':')[0]);
TimeSpan ts = TimeSpan.FromHours(hours);
TimeSpan.Parse() doesn't work directly in this case because it complains (fair enough!) about the 25 in the hour notation.

If you want to code it out... this should be a starting point:
string dateString = "0001-01-01 25:00:00";
string[] parts = dateString.Split(' '); //now have '0001-01-01' and '25:00:00'
string datePart = parts[0]; // '0001-01-01'
string[] timeParts = parts[1].Split(':'); //now have '25', '00', and '00
DateTime initialDate = DateTime.ParseExact(datePart, "yyyy-MM-dd", CultureInfo.InvariantCulture);//use the date as a starting point
//use the add methods to get your desired datetime
int hours = int.Parse(timeParts[0]);
int minutes = int.Parse(timeParts[1]);
int seconds = int.Parse(timeParts[2]);
DateTime resultDate = initialDate.AddHours(hours)
.AddMinutes(minutes)
.AddSeconds(seconds);
Of course, it makes assumptions that the input is formatted properly and is parsable, etc..
In addition, you could definitely use timespan instead of the individual add methods for hour, minute, second as some other answers are..

In case nobody points out an out-of-the-box answer, here is a neat ActionScript class I wrote to parse time inputs (human input)...
https://github.com/appcove/AppStruct/blob/master/Flex/AppStruct/src/AppStruct/TimeInput.as
It would be very simple to port this to C#, and you could tweak the 24 hour logic to result in #days, #hours, #minutes.
Good luck!

You are specifying an invalid date. So not only can you not parse it, you cannot store it!
How about a nice TimeSpan object instead? (It also has a Parse() method.)
Alternatively, use a sscanf()-type function like the one at http://www.blackbeltcoder.com/Articles/strings/a-sscanf-replacement-for-net to extract each number separate. (Best if you have no control over the string format being read.)

How to convert a date of integers to a formated date string (i.e. 2012009 to 2/01/2009)

Any ideas?
I can't come up with any.
I have a list of dates I'm loading in from a csv file and they are saved as all integers, or rather a string of integers (i.e. Jan 1, 2009 = 1012009)
Any ideas on how to turn 1012009 into 1/01/2009?
Thanks!

Since the date is stored as a string, you may want to use ParseExact:
DateTime date = DateTime.ParseExact("28012009", "dMMyyyy", null);
ParseExact will throw an exception if the format doesn't match. It has other overloads, where you can specify more than a single possible format, if that is required. Note that here provider is null, which uses the current culture.
Depending on style you may wish to use TryParseExact.

int date = 1012009;
var month = date / 1000000;
var day = (date / 10000) % 100;
var year = date % 10000;
var formatted = new DateTime(year, month, day).ToString();
This assumes month-day-year; if the numbers are day-month-year, I’m sure you’ll be able to swap the month and day variables to accommodate that.
If you want to customise the date format, you can do so as described in:
Standard Date and Time Format Strings
Custom Date and Time Format Strings

Let 10102009 be dateInt.
string dateString = dateInt.ToString();
int l = dateString.Length;
dateString = dateString.Insert(l-3,"/");
dateString = dateString.Insert(l-6,"/");
You should now have 1/01/2009 in dateString.. You can also try the ParseExact function..