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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 datetime string to Utc

how to convert datetime string to utc time format in GMT.
var x = "02/01/2017 10:00";
var z = DateTime.ParseExact(x, "ddd, dd MMM yyyy HH:mm:ss 'GMT'", CultureInfo.InvariantCulture);

Actually the thing you have tried is wrong, what you can do is, get the DateTime value equivalent to the given date and then convert them to the UTC time, try something like the following:
string inputDateStr = "02/01/2017 10:00";
DateTime inputDate;
if (DateTime.TryParseExact(inputDateStr, "MM/dd/yyyy HH:mm", CultureInfo.InvariantCulture, DateTimeStyles.None, out inputDate))
{
Console.WriteLine("Date time Now : {0} ", inputDate);
Console.WriteLine("Date time UTC : {0} ", inputDate.ToUniversalTime().ToString("ddd, dd MMM yyyy HH:mm:ss 'GMT'"));
}
Working Example

I am not sure what you wanted to do. But, Universal Time, Zulu time, and UTC are effectively modern names for Greenwich Mean Time (GMT). So both the times will be same. You can use the DateTime.ToUniversalTime() or DateTime.UtcNow method to get the UTC time.

It's not completely clear what you are asking... UTC isn't a format, it's a timezone, equivalent to GMT (well, actually that isn't strictly true, but for the purposes of this question it should be ok.
What point in time is your string representing? Is it UTC? Or is it the local time in Moscow? You need to have the answer to that, because they are completely different times.
If the string represents a UTC time, then you can do something like this:
// first parse it to a DateTime object. Notice that the format string corresponds with the string you have - in your code, they were completely different.
var dateTimeWithUnspecifiedKind = DateTime.ParseExact(x, "dd/MM/yyyy HH:mm", CultureInfo.InvariantCulture);
So how you have a datetime object. It has a "Kind" property of Unspecified. So it isn't storing any information to specify that it's supposed to be UTC, rather than Moscow or New York time.
// so we do this...
var dateTimeAsUtc = DateTime.SpecifyKind(dateTimeWithUnspecifiedKind, DateTimeKind.Utc);
That means, "keep the numbers the same, but make a note that the datetime is to be interpreted as a UTC datetime".
Then, if you want to convert it back into a string, you can do something like:
var s = dateTimeAsUtc.ToString("O");
Which will give you a nice representation:
2017-01-02T10:00:00.0000000Z
You are lucky you are interested in UTC. The datetime class can only do UTC, "local" (whatever that is), or "Unspecified". It's not super useful for this sort of thing. DateTimeOffset is a bit better - it's basically a DateTime but it also stores the difference between UTC and the offset that's in force at the time.
That may be all you need. If you ever need real clarity around this stuff though, take a look at Noda time - an alternative set of date and time classes for .NET.

Parsing string into Datetime

This may sound really dumb but I'm struggling with some String parsing into some DateTime
This should be pretty easy with DateTime.ParseExact but the thing is I'm not even able to tell what the pattern is.
string[] patterns = new string[] {"dd/MM/yyyy", "ddMMyyyy", "dd/MM/yyyy HH:mm:ss"};
DateTime test = DateTime.ParseExact(DatMEC, patterns, CultureInfo.InvariantCulture,DateTimeStyles.None);
I used to convert my String to DateTime that way, but in a few cases, I'm facing some parsing error.
I logged the String to check the format, and here is what I've got causing the exception :
String dateException="2/1/2004 12:00:00 AM"
I was wondering what was the appropriate pattern for this?
I tried the following :
"dd/MM/yyyy HH:mm:ss aaa"
"d/M/yyyy HH:mm:ss a"
"M/d/yyyy HH:mm:ss a"
"MM/dd/yyyy HH:mm:ss aaa"
But none of them seems to work.
Am I missing something? I even wonder if it's parseable?

You need to use single day and month numbers d and M and use hh specifier instead of HH since it's 12-hour clock time. Also tt specifier for AM designator.
d/M/yyyy hh:mm:ss tt
Further reading:
Custom Date and Time Format Strings

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))
{
}

DateTime conversion regardless of culture

I have a text string that needs to become a DateTime object:
Feb 10, 2012 at 16:33.29
This text does not change, but the software will run on many different devices with different DateTime formats.
How can I set a custom DateTime parser so that regardless of culture I will get a fully populated DateTimeobject?

parse with CultureInfo.InvariantCulture?

Use ParseExact with a custom format string and the invariant culture:
DateTime date = DateTime.ParseExact(theString, "MMM d', 'yyyy' at 'HH':'mm'.'ss", CultureInfo.InvariantCulture);

Here’s a custom format to match your example:
var dt = DateTime.ParseExact(
"Feb 10, 2012 at 16:33.29",
"MMM d, yyyy 'at' HH:mm.ss",
CultureInfo.InvariantCulture);

One thing has got nothing to do with the other.
DateTime.Parse(value, formatstr) returns a DateTime.
The DateTime does not have a format, unless you want to talk about how it's represented in memory. When you convert it to a string, you generally do it with an implicit or explicit format, once you have it is no longer a datetime...