What is an evergreen countdown timer, and should I use one?

In Products → Countdown Timer you can pick between:

• Fixed end date: everyone sees the same timer counting down to the Ends at date, which suits a real sale like Diwali.
• Evergreen (per-visitor): each visitor gets their own countdown of the length you set, starting from their first visit (it is remembered in their browser).

Use a fixed date for real deadlines. Use evergreen only for offers you are happy to honour for every visitor.