Hi nyzen!
Welcome to our forum and thanks for contacting us with your question!
If I understand you correctly, this date is generated by browsers and it is not related to Highcharts. If I remember correctly, it is generated based on your language and timezone. You can try to look at how to change it on the more general forums like StackOverflow etc.
In case of any other questions, feel free to contact us anytime.
Best regards!