Why do timelines countdown to zero and then count up?

Timelines typically countdown to zero to signify the completion of a specific duration or event, often creating a sense of anticipation or urgency. Once they reach zero, counting up can indicate the elapsed time since the event started or the duration of a project, allowing for easy tracking of progress or performance. This format effectively communicates the transition from a deadline to an active phase, such as an event's duration or post-launch metrics.