Header Date format

In the free Weblog theme the date in the header is formatted in US style, even when I use a different format in the general settings for the site. The words for months are in the native language.
Now I changed the code in the theme but when it is updated the date format is most likely changed to US again.
Should I make a child theme?
Is it possible to copy the date format from the site settings instead of editing the code in the theme?