To handle date, time, and number formatting in a localized GUI:

1. Use locale-aware formatting functions or libraries.

2. Support Unicode's Common Locale Data Repository (CLDR).

3. Utilize internationalization frameworks or libraries.

4. Format dates, times, and numbers according to local conventions.

5. Use locale-specific formatting patterns.

6. Handle different calendar systems (e.g., Gregorian, Islamic).

7. Consider time zone differences.

8. Format numbers with locale-specific decimal and thousand separators.

9. Use locale-aware parsing for user input.

10. Test with different locales to ensure correct formatting.

Some popular libraries and frameworks for handling localized formatting include:

1. ICU (International Components for Unicode)

2. CLDR (Common Locale Data Repository)

3. Java's DateFormat and NumberFormat classes

4. .NET's CultureInfo and DateTimeFormatInfo classes

5. Python's datetime and locale modules

By using these libraries and following best practices, you can ensure that your GUI correctly handles date, time, and number formatting for users worldwide.