Final answer:
The proper discard date to mark on a dish with combined food is the oldest item's use-by date.
Step-by-step explanation:
The proper discard date to mark on a dish with combined food, such as soup made with multiple precooked items, is the oldest item's use-by date.
This means that you should check the use-by dates of all the individual items used in the dish, and mark the dish with the date of the oldest item's use-by date.
For example, if you used chicken, vegetables, and broth to make a soup, and the chicken has a use-by date of January 10th, the vegetables have a use-by date of January 15th, and the broth has a use-by date of January 12th, you should mark the soup with January 10th.