Sidetracked

Open world video games tend to conveniently but unrealistically put the plot on hold while the hero(es) wander around collecting assorted baubles, doing fetch errants for amusing NPCs, or visiting every scenic landmark. Or just grinding out gold to upgrade your belt to +17. Even when the protagonist is literally dying (such as in Cyberpunk 2077 or Baldur’s Gate 3) this time limit is just theoretical. The one exception might be the original Fallout where the ticking clock was actually ticking and could run out. 

Tabletop roleplaying games can do something similar, where the villain’s evil scheme also advances at the speed of plot. The PCs will always arrive at the culmination of the evil ritual. And the cultists will spend exactly as much time preparing for the final encounter as the PCs spend doing…. whatever.

Usually. 

But if the DM gives the adventurers a deadline and a set number of dates, then if they miss that window, things could go badly. The dragon isn’t going to pause its assault. And the evil cult planning on initiating the end of days might finish their ritual, causing the campaign to radically change in tone.