Ok, the native date format is now configurable, see config variable use_browser_date_input in section [web]. The default is 'no'.

There are still some issues with the native date / datetime-local <input> fields in browsers: It is not possible to specify international date format (at least not easily and not server-side). Moreover some browsers leave out seconds when the time given has the seconds part =0 (yukk). This is problematic for use-cases where full seconds are needed even in the case where the seconds part is zero. An application where this matters is my QSO logging program (a QSO is a two-way radio communication between hamradio operators): Some newer digital modes have QSOs ending in 0, 15, 30, or 45 seconds. This means 1/4 of all logged QSO times would be displayed without seconds.

These are the reasons why the configuration default is corrently 'no' for the config variable described above.

I'm closing this issue as fixed.