Microsoft in its new SharePoint Online Modern sites released new webparts. Countdown Timer is one of them.
To add countdown timer in your SharePoint Intranet.
Goto –> Site Contents —> SitePages —-> Create New Site Page . After that Click Edit Page and Click Plus sign and You will see list of new webparts . So from that list identify count down timer and Add.

If you want to know more in detail. Watch my video below demonstration of Count Down Timer Web part.
50% off on SharePoint Training videos. Click below Link to buy
Create stunning designs with share point provisioning service
Wat