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DateTime.UtcNow.ToString("yyyy/MM/dd HH:mm:ss") gives different formats for different users

I assumed that ToString("yyyy/MM/dd HH:mm:ss") will force the string to be formatted with '/', but I can see that every device gets different formats. How can I force it to be saved with '/'?
Good example-
2021/10/06 18:05:53
Strange examples I see in my DB from different users-
2021-10-06 23:48:37
2021.10.12 12:41:42
2021. 10. 06 19:17:23 ('.'+ space after)
2021.10.13 19.18.16
One solution is to replace every -, . and . to /, but this only solves the strange examples I found. What if there are others?

/ in a format string means "the culture-specific date separator". If you want the literal forward-slash, quote it (and the colons, to avoid the use of a custom time separator):
ToString("yyyy'/'MM'/'dd HH':'mm':'ss")
Alternatively - and probably better - use the invariant culture. Not only will that use / as the date separator, but you won't need to worry about a culture having a different default calendar. (It'll always use the Gregorian calendar, which is presumably what you want.)
Even better, use an ISO-8601 format - you're already using a "slightly unusual for humans" format of year-first, so you might as well go the whole hog and go with the standard format for dates and times.
Sample code:
String text = dateTimeValue.ToString(
"yyyy-MM-dd'T'HH:mm:ss",
CultureInfo.InvariantCulture);
This is also the sortable standard date/time format so you can simplify the code significantly:
String text = dateTimeValue.ToString("s");
(That format always uses the invariant culture.)
That's if you really need to format the string at all, though. If you're saving it in a database, I'd advise you to:
Use an appropriate type in the database, e.g. DATETIME
Store it using a parameter (specifying the value just as a DateTime), not formatted text
If you do both of these, you'll avoid oddities like this.

Another solution that I can think of is creating a new function that creates a date
DateTime date= DateTime.UtcNow;
And extracting manually and splitting the date to a few strings (year,month,day,hour,month,seconds)
string year = date.Year.ToString(); string month = date.Month.ToString();...
and building a string out of it in the right format,
string newDate= year + "/" + month + "/" + day + " "+ hour+":"+ minute+ ":"+seconds;
that way I can be sure it's always one format that I'll decide on

How about storing the date as a number, eg unix time - DateTimeOffset.UtcNow.ToUnixTimeSeconds() or (DateTime.UtcNow - DateTime.UnixEpoch).TotalSeconds - it's a lot simpler and cheaper to store a number than a string
Also wanted to point out that the only date I've
seen you store so far is UtcNow (as written in your answer) - fire base does appear to have a solution for that in that you can send ServerValue.TIMESTAMP and it will cause fb to store the unix time as the server sees it.
My take away from this (never used fb) is that that's how they store dates so perhaps it makes sense to follow :)

Short Time with DateTime.ParseExact

I’m trying to parse a time. I’ve seen this question asked/answered here many times but not for this specific scenario. Here’s my code:
var time1 = DateTime.ParseExact("919", "Hmm", CultureInfo.InvariantCulture);
also
var time2 = DateTime.ParseExact("919", "Hmm", null);
both of these throw the same
"String was not recognized as a valid DateTime"
What I want is 9:19 AM.
For further info I also need to parse “1305” as 1:05 PM, this is working fine.
It seems to me I’m using the correct format. What am I overlooking?

I'm not sure there is any format that can handle this. The problem is that "H" can be either one digit or two, so if there are two digits available, it will grab both - in this case parsing it as hour 91, which is clearly invalid.
Ideally, you'd change the format to HHmm - zero-padding the value where appropriate - so "0919" would parse fine. Alternatively, use a colon in the format, to distinguish between the hours and the minutes. I don't believe there's any way of making DateTime parse a value of "919" as you want it to... so you'll need to adjust the string somehow before parsing it. (We don't have enough context to recommend a particular way of doing that.)

Yes, your format is right but since H specifier might be 2 character, ParseExact method try to parse 91 as an hour, which is an invalid hour, that's why you get FormatException in both case.
I connected to microsoft team about this situation 4 months ago. Take a look;
DateTime conversion from string C#
They suggest to use 2 digit form in your string or insert a date separator between them.
var time1 = DateTime.ParseExact("0919", "Hmm", CultureInfo.InvariantCulture);
or
var time1 = DateTime.ParseExact("9:19", "H:mm", CultureInfo.InvariantCulture);

You cant exclude the 0 prefix to the hour. This works
var time1 = DateTime.ParseExact("0919", "Hmm", CultureInfo.InvariantCulture);
Perhaps you want to just prefix 3-character times with a leading zero before parsing.

Much appreciated for all the answers. I don’t have control of the text being created so the simplest solution for me seemed to be prefixing a zero as opposed to adding a colon in the middle.
var text = "919";
var time = DateTime.ParseExact(text.PadLeft(4, '0'), "Hmm", null);

How to validate string is correct date time

I have some data picked up from an excel file.
I want to validate that the user has entered a valid date time string. I have tried to use DateTime.Parse method but found that certain values seem to be accepted.
For example,
If I submit 3.3 as a date time this is accepted by the DateTime.Parse method as a valid date time and outputs 03/03/2012 00:00:00
I want to want to block this. Only allowing the user to enter correctly formatted date times.
So for example a user could supply 03/03/2012 or 03/03/2012 12:30:00 but not values like 01022012 or 3.3.2012
Any Ideas?

You want to use DateTime.ParseExact or DateTime.TryParseExact
This allows you do parse from a date format string of your choice.
http://msdn.microsoft.com/en-us/library/system.datetime.tryparseexact.aspx
Examples here:-
http://msdn.microsoft.com/en-us/library/ms131044.aspx

You can use RegEx to to this. Something like this should help #"\d{2}/\d{2}/\d{4}(\s+\d{2}\:\d{2}\:\d{2})?"

You can handle it on the client side with various jquery plugin/functions like this or a simple Google search can return many other useful results.
if you want to handle it on the server side, (I am not sure on what project you are working) but depending over it you can write your own method/use Regex or Data Annotation MVC.
If you are still having trouble try adding few details about your project such as Language, Architecture etc. that would help more in providing the right solution.
Hope it helps. Thankyou

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.

Storing a short date in a DateTime object

I'm trying to store a shortened date (mm/dd/yyyy) into a DateTime object. The following code below is what I am currently trying to do; this includes the time (12:00:00 AM) which I do not want :(
DateTime goodDateHolder = Convert.ToDateTime(DateTime.Now.ToShortDateString());
Result will be 10/19/2009 12:00:00 AM

DateTime is an integer interpreted to represent both parts of DateTime (ie: date and time). You will always have both date and time in DateTime. Sorry, there's nothing you can do about it.
You can use .Date to get the date part. In these cases, the time will always be 12:00 but you can just ignore that part if you don't want it.

You only have two options in this situation.
1) Ignore the time part of the value.
2) Create a wrapper class.
Personally, I am inclined to use option 1.

A DateTime will always have a time component - even if it is 12:00:00 AM. You just need to format the DateTime when you display it (e.g. goodDateHolder.ToShortDateString()).

Instead of .Now you can use .Today which will not remove the time part, but will only fill the date part and leave time to the default value.
Later on, as others pointed out, you should try to get the date part ignoring the time part, depending on the situation.

You'll always get the time portion in a DateTime type.
DateTime goodDateHolder = Convert.ToDateTime(DateTime.Now.ToShortDateString());
will give you today's date but will always show the time to be midnight.
If you're worried about formatting then you would try something like this
goodDateHolder.ToString("mm/dd/yyyy")
to get the date in the format that you want.
This is a good resource msdn-dateformat

You can also check out Noda Time based off the Java Joda Time library.

DateTime object stores both the date and the time. To display only the date, you would use the DateTime.ToString(string) method.
DateTime goodDateHolder = DateTime.Now;
// outputs 10/19/2009
Console.WriteLine(goodDateHolder.ToString("MM/dd/yyyy"));
For more information on the ToString method, follow this link

You might not be able to get it as a DateTime object...but when you want to display it you can format it in the way you want by doing something like.
myDateTime.ToString("M/d/yyyy") which gives 10/19/2009 for your example.

DateTime is merely a UInt64 with useful and clever formatting wrapped around it to make it appear like a date plus a time. You cannot eliminate the time element.