usability: for an unknown number, what text should I put to clarify that missing data?

I have a number that takes the start time of a task entered by the user, and the user entered the end time of a task, and uses it to calculate the duration of a task.

For example, a user schedules a task, but does not enter its start time, but enters its end time. In this case, I would later ask the user to check that they enter a start time.

However, I have a card that shows the tasks, and some of the predetermined data on that card are elements such as the time a task took and the start and end time.

How would I show these missing numbers to users?

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