The 30 days of game time is intended for Forever. You might have missed the specifics on the product page:
⁴30 Days WoW® Game Time will be added to your WoW® account with the launch of WoW®: Forever.
So the game time itself will kick in with the launch of WoW: Forever. If you need game time prior to that, you’ll need to add a subscription, or otherwise add game time.