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DateTime Format - Any System Datetime.now to "DD/MM/YYYY hh:mm:ss"

I have a program that do several things.
Two of them is read a date from a txt and rewrite a date in the same txt.
The read of the date is a regex expression like:
[0-9]{2}/[0-9]{2}/[0-9]{4} [0-9]{2}:[0-9]{2}:[0-5]{1}[0-9]{1})
The problem is that my regex expression only works in the format
"DD/MM/YYYY hh:mm:ss" and its impossible to make sure my regex expression can match all system datetime formats.
So, I need to make sure my program run's in every system, regardless the system datetime.now.
For that, i thought about format every system datetime.now, at start, to the format mentioned "DD/MM/YYYY hh:mm:ss".
At the moment i have the following code:
Datetime currentDate = DateTime.ParseExact(DateTime.Now.ToString(), "DD/MM/YYYY hh:mm:ss", CultureInfo.InvariantCulture);
However, when running some tests, using a system date in format "D/M/YYYY h:m:s" i get the error:
"String was not recognized as a valid DateTime."
The problem is that if my date, for example, is "9/27/2019 04:26:46"(M/D/YYYY h:m:s) it can't fit in the format i defined.
Any idea?
Thank you in advance!

You need to use the same format string and culture in every place where you convert the DateTime to string as well. In your sample code, you're doing
DateTime.Now.ToString()
This uses the default culture for the thread, and the default format. Unless assigned otherwise, the thread is probably using the local culture info. Instead, you would want to use the same format and the invariant culture:
DateTime.Now.ToString("dd/MM/yyyy HH:mm:ss", CultureInfo.InvariantCulture);
(note the lowercase "dd". "DD" is not a valid format specifier for date times; these things are case sensitive. Also note the "HH", which gives a 24-hour value, rather than 12-hour)
In practice, just using the invariant culture should be enough for persistence. Cultures already include default datetime formats, so unless you have a specific need to use a different format, why not use the default?
Also note that DateTime doesn't have a format. The format only comes into play when you convert from or to a string. That is the place where you need to ensure the same culture and format is used for both sides of the operation (and that's why for persistence, especially for data shared between different users or computers, you generally want to use the invariant culture).

If you need
to make sure my program run's in every system, regardless the system datetime.now
you can adapt international standard for this, say, ISO 8601.
In order to validate the DateTime, regular expressions like you have are not enough (just imagine leap years), but TryParse does it job:
string source = "2019-09-26T23:45:59";
// Either current culture date and time format or ISO
bool isValid = DateTime.TryParse(
source,
CultureInfo.InvariantCulture,
DateTimeStyles.AssumeLocal,
out var _date);
Or if you want to be more restrictive use TryParseExact:
// ISO only
bool isValid = DateTime.TryParseExact(
source,
"s",
CultureInfo.InvariantCulture,
DateTimeStyles.AssumeLocal,
out var _date);
If you want to represent DateTime.Now in ISO 8601, add "s" standard format string:
string dateAsString = DateTime.Now.ToString("s");
Alas, you can provide a bunch of formats which are able to cope with any date and time formats; a classical example of ambiguous date is
01/02/03 - 01 Feb 2003 (Russia)
01/02/03 - 02 Jan 2003 (USA)
01/02/03 - 03 Feb 2001 (China)
You can alleviate the problem, while providing several formats:
// Here we try to support 4 formats (note different delimeters)
string[] formats = new string[] {
"s", // try ISO first
"dd'.'MM'.'yyyy HH':'mm':'ss", // if failed try Russian
"MM'/'dd'/'yyyy HH':'mm':'ss", // on error have a look at USA
"yyyy'-'MM'-'dd HH':'mm':'ss", // the last hope is Chinese
};
bool isValid = DateTime.TryParse(
source,
formats,
CultureInfo.InvariantCulture,
DateTimeStyles.AssumeLocal,
out var date);

Convert any DateTime format to US DateTime format in C#

I want to convert any DateTime format to US DateTime format i.e.
MM-dd-yyyy HH:mm:ss
I have the server date which can be anything like it can have AM / PM added in the tail too. I have to take care of most possible scenarios.
CodeValidTill = DateTime.ParseExact(dateObject.ToString(), "MM-dd-yyyy HH:mm:ss", culture);
I have also tried below method to cover most of the cases:
public static DateTime ConvertToUSDateFormat(string dateString)
{
string[] formats = {"M/d/yyyy", "MMM dd yyyy", "MM-dd-yyyy HH:mm:ss", "M/d/yyyy hh:mm:ss tt", "MM/dd/yyyy hh:mm:ss tt"};
return DateTime.ParseExact(dateString, formats, CultureInfo.InvariantCulture, DateTimeStyles.None);
}
Is there any way that we can write a generalized method to handle such situation?

I have a number of hard and fast rules for dealing with DateTimes:
Always store, retrieve and transmit the UTC value. You do not want to deal with Timezones. That way lies madness
Avoid storing, retrieving or transmitting them as Strings.
If you can not avoid store/retreive/transmit as string, pick a fixed String Encoding and Format at all ends
If you follow all those rules you can somewhat reasonably work with DateTimes without going mad.
If you can not follow those rules, you should simply call it impossible so you can enforce the rules with a proper rework of the faulty code.

Agree with jdweng. Its a really good idea to store you dates as a DateTime. This object is format independent and can account for special cultural formats.
Example
DateTime thisDate = new DateTime(2018, 1, 29);
Console.WriteLine(thisDate.ToString("d"));
This should display 1/29/2018
More info on DateTime formatting with the "ToString" overloads

Can not Parse string to DateTime in Windows Phone 7

I am trying to convert the string to DateTime. But I can not convert.
DateTime dt = DateTime.Parse("16/11/2014", CultureInfo.InvariantCulture);
Console.WriteLine("Date==> " + dt);
The error is FormatException.
My input time format is "dd/MM/yyyy".
Please let me any idea to resolve my problem.

Given that you know your input format, you should specify it with `ParseExact:
DateTime dt = DateTime.ParseExact(text, "dd/MM/yyyy",
CultureInfo.InvariantCulture);
I would always recommend being as explicit as you can be about date/time formats. It makes your intention very clear, and avoids the possibility of getting months and days the wrong way round.
As Soner has stated, CultureInfo.InvariantCulture uses MM/dd/yyyy as its short date pattern, as you can validate with:
Console.WriteLine(CultureInfo.InvariantCulture.DateTimeFormat.ShortDatePattern)
As a mild plug, you might want to consider using my Noda Time project for your date/time handling - aside from anything else, that allows you to treat a date as a date, rather than as a date and time...

Because InvariantCulture doesn't have dd/MM/yyyy as a standard date and time format, but it has MM/dd/yyyy as a standard date and time format.
That's why it thinks your string is MM/dd/yyyy format, but since there is no 16 as a month in Gregorian calender, you get FormatException.
Instead of that, you can use DateTime.TryParseExact method to specify exact format like;
string s = "16/11/2014";
DateTime dt;
if(DateTime.TryParseExact(s, "dd/MM/yyyy", CultureInfo.InvariantCulture,
DateTimeStyles.None, out dt))
{
}

Converting dd/mm/yyyy formatted string to Datetime [duplicate]

This question already has answers here:
Converting a String to DateTime
(17 answers)
Closed 9 years ago.
I am new to DotNet and C#. I want to convert a string in mm/dd/yyyy format to DateTime object. I tried the parse function like below but it is throwing a runtime error.
DateTime dt=DateTime.Parse("24/01/2013");
Any ideas on how may I convert it to datetime?

You need to use DateTime.ParseExact with format "dd/MM/yyyy"
DateTime dt=DateTime.ParseExact("24/01/2013", "dd/MM/yyyy", CultureInfo.InvariantCulture);
Its safer if you use d/M/yyyy for the format, since that will handle both single digit and double digits day/month. But that really depends if you are expecting single/double digit values.
Your date format day/Month/Year might be an acceptable date format for some cultures. For example for Canadian Culture en-CA DateTime.Parse would work like:
DateTime dt = DateTime.Parse("24/01/2013", new CultureInfo("en-CA"));
Or
System.Threading.Thread.CurrentThread.CurrentCulture = new CultureInfo("en-CA");
DateTime dt = DateTime.Parse("24/01/2013"); //uses the current Thread's culture
Both the above lines would work because the the string's format is acceptable for en-CA culture. Since you are not supplying any culture to your DateTime.Parse call, your current culture is used for parsing which doesn't support the date format. Read more about it at DateTime.Parse.
Another method for parsing is using DateTime.TryParseExact
DateTime dt;
if (DateTime.TryParseExact("24/01/2013",
"d/M/yyyy",
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out dt))
{
//valid date
}
else
{
//invalid date
}
The TryParse group of methods in .Net framework doesn't throw exception on invalid values, instead they return a bool value indicating success or failure in parsing.
Notice that I have used single d and M for day and month respectively. Single d and M works for both single/double digits day and month. So for the format d/M/yyyy valid values could be:
"24/01/2013"
"24/1/2013"
"4/12/2013" //4 December 2013
"04/12/2013"
For further reading you should see: Custom Date and Time Format Strings

use DateTime.ParseExact
string strDate = "24/01/2013";
DateTime date = DateTime.ParseExact(strDate, "dd/MM/yyyy", null)
DateTime.ParseExact
null will use the current culture, which is somewhat dangerous. Try to supply a specific culture
DateTime date = DateTime.ParseExact(strDate, "dd/MM/yyyy", CultureInfo.InvariantCulture)

You can use "dd/MM/yyyy" format for using it in DateTime.ParseExact.
Converts the specified string representation of a date and time to its
DateTime equivalent using the specified format and culture-specific
format information. The format of the string representation must match
the specified format exactly.
DateTime date = DateTime.ParseExact("24/01/2013", "dd/MM/yyyy", CultureInfo.InvariantCulture);
Here is a DEMO.
For more informations, check out Custom Date and Time Format Strings

Unable to parse date time in C#

I know such questions are in ton here. Please go through once
I have a string coming from textbox having current date e.g. 10/9/2012, my class property is of DateTime? type. I am using Convert.ToDateTime(datetime_string_from_textbox) but it gives me a FormatException. I then tried DateTime.ParseExact(string, format, CultureInfo, DateTimeStyle) as suggested by Jon Skeet here but still it gave me the same exception.
One more thing — my local machine date time format is dd-mm-yyyy. When I switch this to mm/dd/yyyy format the code works fine. So basically , I want to know how to parse a valid datetime string to a DateTime object irrespective of the regional settings, or any settings or any dependency on local machine.
Is this possible?
Update : Code in use
employee.JoiningDate = DateTime.ParseExact(string.Format("{0} 00:00:00", JoiningDate.Text.Trim()), "MM/dd/yyyy hh:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal | DateTimeStyles.AssumeUniversal);
Existing Problem and Required Solution
My system datetime shows 24-10-2012 (that is, 24th Oct) and I have 10/17/2012 in my text box (that is, 17th Oct) since the text box date is also valid and after deployment again the client datetime format will become unknown so, I want a generic way to parse any valid datetime string irrespective of regional settings. Is this possible?

This should work:
var date = DateTime.ParseExact(str, "M/d/yyyy", CultureInfo.InvariantCulture);
As tested bellow: 

Try formatting your date to international date format using this method:
How would you format DateTime in international format?
Also you can check this for your current culture:
Set Default DateTime Format c#

It totally depends on the machine settings. DateTime.ParseExact(str, "dd/MM/yyyy", CultureInfo.InvariantCulture); will work for British format but it will give format exception on US format. So use format according to your machine settings.

Try the following if it works
var formatInfo = new DateTimeFormatInfo();
formatInfo.ShortDatePattern = "MM/dd/yyyy";
DateTime.Parse(date, formatInfo);