Description: LibreOffice provides 9 different calendar types as listed here: (some are only different display names) $ grep i18npool_Calendar_ i18npool/util/i18npool.component constructor="i18npool_Calendar_ROC_get_implementation"> constructor="i18npool_Calendar_dangi_get_implementation"> constructor="i18npool_Calendar_buddhist_get_implementation"> constructor="i18npool_Calendar_gengou_get_implementation"> constructor="i18npool_Calendar_gregorian_get_implementation"> constructor="i18npool_Calendar_hanja_get_implementation"> constructor="i18npool_Calendar_hanja_yoil_get_implementation"> constructor="i18npool_Calendar_hijri_get_implementation"> constructor="i18npool_Calendar_jewish_get_implementation"> They are: 1. ROC 2. Dangi 3. Buddhist 4. Gengou 5. Gregorian 6. Hanja 7. Hanja Yoil 8. Hijri 9. Jewish These are implemented in LibreOffice in i18npool/source/calendar in 1.5 LOC in C++. On the other hand, some widely used calendar systems exist that are not supported in LibreOffice. For example, request for implementing Jalali (Persian) calendar system is filed as tdf#147660. The good news is that ICU provides cross platform implementation for 18 calendar types, either calendar systems or alternative display names: https://github.com/unicode-org/icu/blob/main/docs/userguide/datetime/calendar/index.md https://deepwiki.com/unicode-org/icu/5.1-calendar-systems List of 18 supported calendar types can be seen in: icu4c/source/i18n/calendar.cpp: static const char * const gCalTypes[] = { "gregorian", "japanese", "buddhist", "roc", "persian", "islamic-civil", "islamic", "hebrew", "chinese", "indian", "coptic", "ethiopic", "ethiopic-amete-alem", "iso8601", "dangi", "islamic-umalqura", "islamic-tbla", "islamic-rgsa", nullptr }; The ICU library is already part of the external libraries that are linked to LibreOffice. The task here is to replace the internal implementation with the one provided by ICU. The above file is actually available as below, and can be simply used in i18npool module of LibreOffice: workdir/UnpackedTarball/icu/source/i18n/calendar.cpp (~3K LOC C++) Some code refactoring and restructuring will be needed to provide support for additional calendar systems provided by ICU. Then, locales should be also modified to use appropriate calendars, which were not available before. Please note that macOS and Windows provide their own implementation of calendar systems, which can be set in their locale settings and is available in their native API. But, those native APIs are not relevant here.