Btw I look at this: https://support.mozilla.org/en-US/kb/customize-date-time-formats-thunderbird#w_choose-thunderbird-date-and-time-formats-defined-by-operating-system-or-application-locale

But in Thunderbird there is no Preferences in the hamburger menu

do you use the English version of Thunderbird or the dutch? there is normally a preference menu item in the hamburgermenu. In the latest versions is the menu Language & Appearance not available anymore but i believe that changing the setting in the about:config still works

This is correct: https://support.mozilla.org/en-US/kb/customize-date-time-formats-thunderbird

There is a wanted to change the date format I did as suggested and changed the OS language. This also did change the menus in Thunderbird. Finally I was able to set the date settings in Instellingen > Algemeen > scoll down > click Configuration-editor and add intl.date_time.pattern_override.date_short as string and specify the proper date format