指定した日まであと何日残ってる？

manifest.json

{
  "update_url": "https://clients2.google.com/service/update2/crx",
  "background": {
    "scripts": [
      "background.js"
    ],
    "css": [
      "css/style.css",
      "css/bootstrap.min.css"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_popup": "options.html",
    "default_icon": {
      "19": "icon/19.png"
    }
  },
  "icons": {
    "16": "icon/16.png",
    "48": "icon/48.png",
    "128": "icon/128.png"
  },
  "description": "指定した日までの残り時間（残りの日数・残りの週数・残り％）をChromeタブに表示します。",
  "manifest_version": 2,
  "options_page": "options.html",
  "name": "指定した日まであと何日残ってる？",
  "permissions": [
    "alarms",
    "storage"
  ],
  "version": "2.0.2"
}