That message is from JavaScript, not a PHP setting. Since you didn't include a URL to your site in the post or in your profile, I can't tell what weather software you're using.
The script you want to edit to use the 2-digit date is named ajax{XX}wx.js -- they all have similar construction, just use different sources of realtime data. Let me know the URL of your website, and I can be more specific with modification instructions.