For an unknown number, what text should I put to make it clear that data is missing?

I have a number that takes in the user inputed start time of a task, and the user inputed end time of a task, and uses that to calculate how long a task took.

Say, a user schedules a task, but doesn't enter their start time, but enters their end time. In this case I would later prompt the user when reviewing to enter a start time.

However I have a card that shows tasks, and some of the default data in that card are items such as how long a task took, and the start-time and end-time.

How would I go about showing these missing numbers to users?

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