Both sentences are correct but the use depends on the time frame they are referring to. If you use the past tense (declared) then you are probably referring "earlier today, this morning" and it is now afternoon. So the action is completely finished. If you use present perfect (has declared) then you are implying that the action has not yet finished and something may still follow. But be careful: if you use "he was going" then you also indicate that he has changed his mind in the mean time and he is actually *not* going to leave the company. So you are actually only signalling his initial intent in the past.

Caution: I might be wrong 1st statement ( Present Perfect ) is giving the impression that he just declared while Simple past states that action was done a little time ago. I have given you answer that no gave you.

Hi Leka - As I see it, I agree with Akshit; by using Present Perfect tense, you are stating that you have the EXPERIENCE. Present Perfect: "Have you (ever) been to Europe? Yes, I have been to Europe." This has a totally different feeling than, Simple Present: "Did you go to Europe? Yes, I went to Europe (last fall.)" The first one is not specific to the time, but just let's the other person know that you did it sometime in the past. The second can be more specific. Often, with Simple Present you can ask and reply by telling 'when' you did something. To answer your question, I would be surprised if someone used the Present Perfect in a sentence with the word "Today." It sounds a bit unnatural. Go for the Simple Past, it's less awkward.