Why is the month abbreviated differently when the date format includes a slot for days as opposed to when it doesn’t?

Question

When running a .NET 8 console application, why does MMM show "June" instead of "Jun" when its part of a date format (e.g. .ToString("dd-MMM-yy")) but "Jun" (e.g., .ToString("MMM")) when its by itself?

// Set culture to English (Australia)
var culture = new CultureInfo("en-AU");

// Get the current date
var currentDate = new DateTime(2024, 6, 8);

// Display the current date using the short date pattern
var formattedDate = currentDate.ToString("d", culture);
Console.WriteLine("Short date in en-AU culture: " + formattedDate); // Outputs: 8/06/2024

// Display the abbreviated month name separately
var abbreviatedMonth = currentDate.ToString("MMM", culture);
Console.WriteLine("Abbreviated month: " + abbreviatedMonth); // Outputs: Jun

var incorrect = currentDate.ToString("dd-MMM-yy", culture);
Console.WriteLine("Incorrect format: " + incorrect); // Outputs: 08-June-24

Windows gets it right, but not C#. Note the month in the bottom right of the screenshot (I changed my Windows time to be in the month of June).

Answer 1

There are two abbreviations for months in DateTimeFormatInfo, one is called AbbreviatedMonthNames and the other is called AbbreviatedMonthGenitiveNames. The abbreviation of June in AbbreviatedMonthNames is Jun and in AbbreviatedMonthGenitiveNames it is June.

The explanation of genitive month names in the document is as follows:

In some languages, a month name that is part of a date appears in the genitive case. For example, a date in the ru-RU or Russian (Russia) culture consists of the day number and the genitive month name, such as 1 Января (1 January).

The key code for which abbreviation to choose is here: IsUseGenitiveForm, and its comment says:

Action: Check the format to see if we should use genitive month in the formatting. Starting at the position (index) in the (format) string, look back and look ahead to see if there is "d" or "dd". In the case like "d MMMM" or "MMMM dd", we can use genitive form. Genitive form is not used if there is more than two "d".

So if there is d or dd in the format, June will be chosen.