Adam was suppose to die the day that he ate the fruit.

It just means a marked period of time.

That is what our earth days are.

The time it takes for the earth to do a full spin.

This is not the measurement for all days ever mentioned and in the universe.

Mars for a start has a different length day.

In the creation day when plants were created. They flowered and bore fruit.

So definitely not 24 hours.