Examine source code of Date Calculator

{
  "manifest_version": 2,
  "name": "Date Calculator",
  "version": "1.0",
  "description": "Displays dates 28, 14, 7, and 4 days before and after today.",
  "permissions": [
    "<all_urls>"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icon-16.png",
      "48": "icon-48.png",
      "128": "icon-128.png"
    }
  },
  "icons": {
    "48": "icon-48.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{9f872384-450c-40da-a8cd-675f286d70e2}"
    }
  }
}