The Hollywood star will celebrate the milestone in December (10) and he's treating his family to an adventure in the Arctic Circle to scuba dive with walruses.
But Gyllenhaal admits there's another reason behind the icy excursion.
He tells the New York Daily News, "The only reason I'm doing it is I wanted to do something that absolutely terrified me - killer whales and walruses and freezing cold water? That seems absolutely terrifying."
And the actor insists he's not worried about reaching the landmark age after a chat with his sister Maggie Gyllenhaal.
He says, "I was talking to my sister the other day about being 30 and celebrating the life you've had up to this point. I feel there's a massive sort of sea change coming in myself and I'm psyched."