The variables doing that display in WEEWXtags.php are $mcoldestdayonrecord = '17,4C on: 17 maj 2021';
$mcoldestnightonrecord = '8,4C on: 17 maj 2021';
$mwarmestdayonrecord = '17,4C on: 17 maj 2021';
$mwarmestnightonrecord = '9,9C on: 16 maj 2021';
The result of 1/1/1970 is due to 'maj' instead of 'May' in the data.
Have you setup Settings.php to have $SITE['monthNames'] = array( // for wxastronomy page .. replace with month names in your language
"januar","februar","marts","april","maj","juni",
"juli","august","september","oktober","november","december");
PHP (by default) expects the month names to be in English, and your data appears to have Danish month names.. add the above to Settings.php to replace the English default, and the dates should display correctly there and on the wxastronomy.php page too.