nablar

Thank you for your answer, that’s exactly what I needed !

It is actually a common issue :
https://support.advancedcustomfields.com/forums/topic/date-picker-fields-in-repeaters-gone-wrong-after-wp-5-3-update/

Date/Time component improvements in WordPress 5.3

Hey frannyj09,
Actually the issue is with the date_i18n function of wordpress. The recent update (WordPress 5.3) introduced this problem, when they converted the function in a wrapper for the wp_date function.
In short : the plugin authors are not responsible for this bug, WordPress is (which is kinda worse actually).

If you have a call to the date_default_timezone_set function in your functions.php file, try deleting it : that fixed it for me.