The gameplay begins with the player controlling a small heart inside a battle arena. This heart represents the player's soul and must be moved around the screen to avoid incoming attacks. Sans launches a variety of attacks that can appear from different directions, forcing players to constantly pay attention to the arena. Spikes, bones, projectiles, and other hazards can quickly overwhelm an inexperienced player.

One of the most important mechanics in bad time simulator is movement. Players must guide the heart through narrow spaces while avoiding obstacles. Some attacks require the player to move horizontally or vertically, while others demand careful positioning and quick changes in direction. The attacks are often designed to work together, meaning that surviving one pattern does not necessarily mean the danger is over. Players need to anticipate what will happen next instead of simply reacting to what is already on the screen.