The present perfect is used in the first sentence because the speaker doesn't remember when the event happened. And then he/she used the simple past but shouldn't : the simple past is only used when you want to express when the action happened, which clearly isn't the case in this sentence as the speaker doesn't remember when it happened.

The first sentence expresses some news and the second gives details. The first sentence is in the present perfect and the second in the past simple/past continuous. When we tell somebody about something new that happened in the past, we should first use the present perfect, then we use the past simple/past continuous to give details. 

The sentences are in the past simple. The first one expresses some news, and the second gives details. 

The present perfect and the past simple could be used interchangeably in this sentence.