usability – For an unknown number, what text should I use to indicate that data is missing?

I have a number that takes the start time of a task entered by the user, and the end time of the task entered by the user, and l & # 39; Used to calculate the duration of a task.

Let's say that a user schedules a task, but does not enter the start time, but enters the end time. In this case, I would later ask the user when checking to enter a start time.

However, I have a card that shows the tasks, and some of the default data on this card are items such as the duration of a task, the start time, and the hour. the end.

How could I show these missing numbers to users?

I played with "N / A" and "?" and even showing the real time (in this case, "0").