You can use this app to create countdown timer: Metafield Guru
+ After install app, go to App and config for it.
+ Choose the “Product & Variants”
+ Then choose the Product
+ You can set up for all the variants or one of them
If want show countdown on page single product, you need to enter value date time for it.
1. Namespace: event
2. Key: countdown
3. Value: value (yyyy/mm/dd)
Hope you success!!