In the Tumblr Editor, open your Account Settings and select Edit Appearance.
On the next page, select Edit Theme.
Click on the Edit HTML link.
Paste this exact script into your Tumblr HTML file. It's best to add it between the Head tags. Then click on Update, and save.
Create Text Post
Return to your Tumblr Admin page, and create a new Text post.
Paste this shortcode into the post and save. If you expand the post, you will see your new POWr Birthday Countdown. You can edit POWr Birthday Countdown by clicking the POWr Settings Icon in the corner.
Edit your Birthday Countdown on Tumblr:
Go to your live Tumblr web page and click the Settings Icon in the upper right corner of your Birthday Countdown Plugin.