Helen, you might want to look at this for a general description of how the Present Perfect tense works. The idea behind the tense is that something happened at an unspecified time before now. So the issues like "I haven't seen Mr. Whitford" (for the whole life of me) or "I have seen this movie once or twice" represent the classical usage of the tense. However, as it's already mentioned in the description on the link, you may want to specify the period of time, during which something has happened. You may use such expressions as "in the last year", "in the past few weeks", "this week", et cetera. All those expressions of time do include today in them. Like in Russian "в этом году", or "последние несколько дней" and so on. But if the period of time ended before today, as in "last year", "yesterday", "three years ago", you may not use the Present Perfect tense. But we have also mentioned that today is included into the acceptable period of time. That said, you may as well use it to limit the period of time you are looking in for an event. But I don't think it's normal in colloquial speech to refer to things happened today using the Present Perfect tense. On the other hand, I wouldn't call it ungrammatical either (I hope native English speakers don't mind my saying that). But it conveys a somewhat different sense than just uttering that something happened today. You might want to take a look at this short discussion, I hope it can elaborate.

Sincerelly yours, Eugene