"It is open until 8 pm."
The verb "to open" in Japanese:
Trans. 1: 開ける【あける】
Trans. 2: 開く 【ひらく】
Intrans. 1: 開く【あく】
Intrans. 2: 開く【ひらく】
As you can see, the transitive and the intransitive forms of the verb have two readings each. A quick Google search tells that the 1. readings are more commonly used in conversations, whereas the 2. readings are more used in literature. The 2. readings are the same for transitive and intransitive.
*A transitive verb always has a direct object, the action is being done "to" something. An intransitive verb does not have an object, the action is only affecting the person/thing doing it.
I close the door. (Trans.)
The store closes soon. (Intrans.)*
The 午後 is really unnecessary here! And it keeps causing me problems sigh I feel like depending on the context it can be omitted and not loose any meaning. I rarely use 午後 and 午前 except for maybe at work when we're scheduling a meeting if it's not immediately obvious from context or for clarification.
In English "open" functions as an adjective in this context, while Japanese uses the verb 開く in its て-form with いる to show that it has opened and is still in that state.