Why doesn't DateTime.ToShortTimeString() respect the Short Time format in "Regional and Language Settings"?

Question

I have run into an issue that is probably due to my mis-understanding of how the DateTime.ToShortTimeString() method works. When formatting time strings with this function, I was assuming that it would respect the "Short Time" setting in Windows 7's Format settings

Control Panel -> Clock, Language and Region -> Region and Language -> Formats Tab.

However .NET seems to select a short time format not based upon this setting but based upon the current culture:

Region and Language -> Location -> Current Location

I did some testing on Windows 7 RC:

Culture: en-GB, 6AM: 06:00, 6PM: 18:00 // HH:mm (United Kingdom)
Culture: en-GB, 6AM: 06:00, 6PM: 18:00 // hh:mm (United Kingdom)
Culture: en-US, 6AM: 6:00 AM, 6PM: 6:00 PM // HH:mm (United States)
Culture: en-US, 6AM: 6:00 AM, 6PM: 6:00 PM // hh:mm (United States)
Culture: el-GR, 6AM: 6:00 πμ, 6PM: 6:00 μμ // HH:mm (Greece)
Culture: el-GR, 6AM: 6:00 πμ, 6PM: 6:00 μμ // hh:mm (Greece)

I used el-GR as that was the culture that the user that reported the problem with, he also tested this on Vista SP2 and Win 7 RC with the same result.

The question is two-fold really: 1) What is my misunderstanding of .NET and Windows Formats? 2) What is the best solution to create a short format time string (HH:mm or hh:mm tt) based upon the operating system, ideally this should work in Mono so I would prefer to avoid reading from the registry or P/Invoke.

Method used to generate the above, for future reference and testing.

[STAThread]
static void Main(string[] args)
{
    CultureInfo culture = CultureInfo.CurrentCulture;

    DateTime sixAm = new DateTime(2009, 07, 05, 6, 0, 0); // 6AM 
    DateTime sixPm = new DateTime(2009, 07, 05, 18, 0, 0); // 6PM

    string sixAmString = sixAm.ToShortTimeString();
    string sixPmString = sixPm.ToShortTimeString();

    string format = "Culture: {0}, 6AM: {1}, 6PM: {2}";

    string output = String.Format(format, culture, sixAmString, sixPmString);
    Console.WriteLine(output);
    Clipboard.Clear();
    Clipboard.SetText(output);

    Console.ReadKey();
}

Update: Based upon Mike's comments below I adapted the above method with the following changes:

The following two lines

string sixAmString = sixAm.ToShortTimeString();
string sixPmString = sixPm.ToShortTimeString();

Changed to

string sixAmString = sixAm.ToString("t", culture);
string sixPmString = sixPm.ToString("t", culture);

I also changed the culture variable to use CultureInfo.CurrentUICulture.

This unfortunatly didn't work as well as I had hoped, the output regardless of the configuration of Short Time in Windows 7's Formats tab was:

Culture: en-US, 6AM: 6:00 AM, 6PM: 6:00 PM

It seems the CultureInfo.CurrentUICulture is always en-US.

Answer 1

Answer to your second question is

DateTimeFormat.Format(DateTime.Now, "t", CultureInfo.CurrentUICulture);

or

DateTime.Now.ToString("t", CultureInfo.CurrentUICulture);

It’s actually always better to use explicit methods that accept CultureInfo. There is no consistency how .Net chooses what to use by default either CurrectCulture or CurrentUICulture or InvarinatCulture.

To make answer full. Also I will outline differences between cultures.

So CurrentCulture is "Control Panel -> Clock, Language and Region -> Region and Language -> Formats Tab." This is culture that you expect your calculations to be. For example you can do your accounting in US, so you have this to be configured in US.

CurrentUICulture is "Region and Language -> Display language", means that when you are emigrant from Ukraine, you want your application to be localized in UA (but all calculations is still in US).

And InvariantCulture is so called culture agnostic locale. You should use this for storing information and so on. Effectively it En-US.

Note: I could be wrong where each setting is located in windows. But you probably got an idea.

Answer 2

In answer to each of my questions:

1) What is my misunderstanding of .NET and Windows Formats?

The short answer is, there is no link between the "Short Time" setting in "Regional and Language" settings and .NET's ShortTimePattern property. However the LongTimePattern property is dictated by the "Long Time" setting.

I adapted the above method replacing the two formatting lines to:

string sixAmString = sixAm.ToString("T", culture.DateTimeFormat);
string sixPmString = sixPm.ToString("T", culture.DateTimeFormat);

Here is the output:

Culture: en-GB, 6AM: 06:00:00, 6PM: 18:00:00 // HH:mm:ss
Culture: en-GB, 6AM: 06:00:00 AM, 6PM: 06:00:00 PM //hh:mm:ss tt

The bottom of this article explained the problem to me.

2) What is the best solution to create a short format time string (HH:mm or hh:mm tt) based upon the operating system setting?

I don't know about the best solution, but I created the following function that converts the LongTimeFormat to a ShortTimeFormat thus allowing an application to follow the users option if they change the "Long Time" (albeit it won't track the "Short Time" setting).

static string GetShortTimeString(DateTime ShortTimeString)
{
    DateTimeFormatInfo dateTimeFormat = CultureInfo.CurrentCulture.DateTimeFormat;
    string ShortTimePattern = dateTimeFormat.LongTimePattern.Replace(":ss", String.Empty);
    ShortTimePattern = ShortTimePattern.Replace(":s", String.Empty);

    return ShortTimeString.ToString(ShortTimePattern);
}

The output after making the above changes:

Culture: en-GB, 6AM: 06:00, 6PM: 18:00
Culture: en-GB, 6AM: 06:00 AM, 6PM: 06:00 PM

The P/Invoke option is to use GetTimeFormat passing the TIME_NOSECONDS using DateTime.ToString(Format) as above. I have not tested this as I would prefer to avoid using P/Invoke.