DateTime.Compare method is not working with different languages

Question

I need to compare two DateTime values in my DotNetNuke module, like this:

// This value actually comes from the database.
DateTime time1 = Convert.ToDateTime("6/2/2013 5:21:05 PM");

// Say it is now "5/31/2013 2:20:33 AM"
DateTime now = DateTime.Now;

int num = DateTime.Compare(time1, now);

As you can see, num == 1 because the DateTime value of time1 is greater than the DateTime value of now. But when I change the language to German (de-DE) in the DotNetNuke settings, the value of now becomes - 31/5/2013 2:20:33 AM. The result is that num becomes -1, meaning time1 is earlier than now. This is incorrect because time1 is actually later than now.

The time1 value is always in mm/dd/yyyy format because it is coming from the server.

So, how do I get the right comparison in case another language is set in DotNetNuke? When I searched the web, I found that I can correct the time with the following code:

provider = new CultureInfo("en-US");
DateTime.ParseExact("5/31/2013", "d", provider);

Is that correct? Is there a better way to handle this concern?

Edited

forgot to say that time 1 comes from my website server before saving to db and is compared with client's website's datetime value in now variable.all codes is my app which is in that client site.these codes at first accessed time1 date by .net methods of getting remote site data.so when that dnn site language is set as de-DE look 'now' variable value is day first which is reverse from my site date is US format month first.in consequence when comparison takes place, my server date is resulted as earlier than client site date! And everything breaks.FYI my server date came as a i week later date which is 2nd june for license purpose.

Edited again:

Hi every1.I found a solution by the following codes.This date comes from my another server as i said above - "6/15/2013"(changing it was bef4 6/2/2013).It's in string format, why it's a string? it's a long history.just let it be a string right now.before converting to string it was in US date format(month before).Now in current website where my app is, these codes seem working.Hope i am not wrong, please confirm

CultureInfo provider = CultureInfo.InvariantCulture;
provider = new CultureInfo("en-US");
DateTime time1 = DateTime.ParseExact("6/15/2013", "d", provider);                    
DateTime now = DateTime.Now;
int num = DateTime.Compare(time1, now);

"num" returning 1 which is right whether dnn site language is German or English.

Answer 1

As you pointed out, there are cultural differences to how we treat dates when they are represented by strings. The DD/MM/YYYY format is common throughout much of Europe and other parts of the world, while MM/DD/YYYY is used almost exclusively by the USA (with a few exceptions). There are plenty of other formats as well.

There is one important format that you should know, which is the ISO8601 standard. You may typically see this as a timestamp, such as 2013-05-31T02:20:33. Sometimes a trailing Z is included to indicate that the time is at UTC. You might also see an offset such as -07:00 or just -0700. This is all part of the same standard. The most common usage is also denoted in RFC3339. Whenever you actually need to send a DateTime as a string, you should probably be using this format. (The exception is when showing the string to a user, and when gathering input from a user.)

Now, you should understand that a DateTime is not a string value. It is internally represented by a 64-bit integer. It does not internally keep any format or culture settings. Conversion between DateTime and string should only be done for purposes of input and output. Usually this is in your user interface, or in the case of ASP.Net, it can be on your web page.

Anywhere else that you work with DateTime, you should not be dealing with strings at all. Especially when comparing values. Let us dissect your code. First we had this line:

DateTime time1 = Convert.ToDateTime("6/2/2013 5:21:05 PM");

Now you said it was coming out of the database, but if that is the case then you shouldn't have a string. Let's assume you are using a SQL Server database (but the same concept applies in others), and the data is in a column of DateTime type. When you retrieve the value from your data access layer, it should flow from SQL to .Net as a pure DateTime. There is no string involved. Let's just assume you are using ADO.Net and you have a DataReader. You should be loading your value as:

DateTime time1 = (DateTime)reader["columnname"];  // good

If instead you are doing something like this:

DateTime time1 = Convert.ToDateTime(reader["columnname"].ToString());  // bad!

then you are introducing culture info into the string that you are using as the intermediate step. This is likely the source of your problem. Don't use a string when you don't need to.

Also, I would avoid using Convert.ToDateTime at all. If you really do need to parse a string, you should use DateTime.Parse, and if you already know the exact format of that string then you should use DateTime.ParseExact.

Next line from your code was:

DateTime now = DateTime.Now;

For this, you should understand that you are taking the local time from the clock on your server. This is probably not the time of the user of your site. And since you are comparing it to a database value, that means you are storing local times in your database as well. This can be very problematic, because the server could have particular time zone settings. Also, if the time zone goes through changes for Daylight Saving Time (a.k.a. Summer Time), then there are some values that don't exist and some values that exist twice. This can cause major issues, especially when comparing these values. The better approach would be to use either DateTime.UtcNow, or use the DateTimeOffset type instead of the DateTime type. You can read more about this, here.

Last line:

int num = DateTime.Compare(time1, now);

This is fine, but you might find it easier to use regular boolean comparisons, such as:

if (time1 >= now) ...

So how then to deal with culture issues? When parsing input from the user, do something like this:

// Use the culture setting of your user.  Perhaps DNN has this already.
CultureInfo provider = new CultureInfo("de-DE");

// Parse the input string, but let the provider do the work to know the format.
DateTime dt = DateTime.Parse("31/5/2013", provider);

// Send data back to the user with the same provider.
string s = dt.ToString("d", provider);

You may prefer to use the TryParse method, in case your user gives bad data:

DateTime dt;
if (!DateTime.TryParse(s, provider, DateTimeStyles.None, out dt)
{
    // handle invalid data
}

Lastly, you should familiarize yourself with DateTime.Kind, and understand the difference between Local, Utc, and Unspecified kinds. And if you find you need to deal with other time zones in your application, then you may also need to learn about the TimeZoneInfo class.