Learn Russian: Мне это давно известно.

Both mean "I have known this for a long time", but Мне это давно известно sounds slightly more formal, objective, or emphatic. While знать focuses on the person doing the knowing (I know it), the short adjective известно (known) focuses on the fact itself (This is known to me).

Why is the pronoun я in the dative case (мне)?

Because известно is a short-form adjective meaning "known", it requires the dative case to express to whom the information is known. The literal translation is "This is known to me for a long time." The grammatical subject of the sentence is actually это (this).

English uses the present perfect "have known," but there is no past tense verb here. How does Russian convey this?

In Russian, when a state or action started in the past and is still continuing in the present, you use the present tense along with an adverb of time like давно (for a long time). Since the "is" (verb to be) is dropped in the present tense, the combination of давно and the present state известно naturally translates to "have known for a long time."

Could I use знакомо instead of известно here?

Not usually for facts. Известно is used for facts, news, or pieces of information. Знакомо (familiar) is used when you are acquainted with a person, a place, or a feeling. Since это here refers to a fact or situation, известно is the correct choice.