Most of the work needs no storefront at all
The editor live preview uses the same rendering code as the storefront, so what you see while you design is what shoppers get. It also acts out what happens at zero: a timer set to hide shows dimmed and marked as hidden for the buyer rather than vanishing from your screen. Get the design and the end behaviour right there before you publish anything.
Working on a duplicate theme
The app embed and the app block are theme settings, so each theme has its own. A published timer shows on every theme where the embed is on, including your live theme. If your live theme already has the embed on, a timer you publish is in front of shoppers straight away.
To try a timer out of sight, duplicate your live theme, turn the embed on in the copy only, publish the timer, and use the theme preview link to look at it. The app Home will still say the embed is off, because its status reads your live theme. That is expected while you work on a copy.
Development stores
On a Shopify Partner development store, the app bills in test mode, so a development store is never charged. That makes it the place to try the paid timer types, the cart and email timers included, before you decide on a plan for a real store.