Deadline

A deadline is a time by which some task must be completed. It is imposed by an authority
Very often, it reflects a time limit that is imposed by a greater authority - for instance, a teacher must have all homework in by a certain time to be able to report fairly to his or her principal that every student had the same chance to do the work.

Deadlines may also be set by a time horizon that comes from something that is not a human authority, but part of nature. For instance, by sunset one must do those tasks requiring daylight. However, a human must observe the sun and decide what light is strong enough to still be daylight, so time limits will still be involved even if one observes a horizon and sets a deadline oneself.

A simple way to remember this is that a time horizon is like the physical horizon where sunset happens, a time limit is a thing people set up to deal with this, and a deadline is a thing powerful people set up to ensure less powerful people comply with the way they deal with it.