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. Update, then save.
Create Text Post
Return to your Tumblr Admin page, and create a new Text post.
Paste this shortcode into the post and save. Expand the post to see your new Countdown Timer plugin. Click the POWr Settings Icon to open the POWr Editor.
Edit your Countdown Timer on Tumblr:
Go to your live Tumblr web page and select the gear icon in the top right corner of your Countdown Timer Plugin.