Tim Cook's iPhone anniversary tweet has a hidden detail only Apple nerds will notice

Tim Cook just acknowledged the iPhone's tenth anniversary in the most Apple way possible.

"Here's to the #iPhone that changed the world, to the man who dreamed it & the people at Apple who have never stopped looking to its future," the CEO wrote.

That may sound like a pretty straightforward sentiment given the occasion, but there's more to the tweet than meets the eye. The tweet was sent at exactly 9:41 a.m., which careful observers will notice matches up with the time on the image of the original iPhone in Cook's tweet. That's not a coincidence.

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You see, it all started with that first iPhone.

When Steve Jobs announced the first iPhone, his keynote onstage at Macworld kicked off at 9 a.m., but it wasn't until 9:42 a.m., when he actually unveiled the iPhone. That time has carried special meaning for Apple ever since.

That's why early Apple ads tended to show an iPhone with the time 9:42 a.m. Former Apple executive Bob Borcher confirmed as much years ago.

Jobs was famous for his exhaustive keynote prep so that time was far from a coincidence, of course. The thinking was, according to former exec Scott Forstall, that waiting until about 40 minutes into a keynote to make "the big reveal" was the most ideal time.

Here's Forstall's explanation, as reported by The Atlantic.

We design the product launch keynotes so that the big reveal of the product happens around 40 minutes into the presentation. When the big image of the product appears on screen, we want the time shown to be close to the actual time on the audience’s watches. But we know we won’t hit 40 minutes exactly. And for the iPhone, we made it 42 minutes. It turned out we were pretty accurate with that estimate, so for the iPad, we made it 41 minutes. And there you are—the secret of the magic time.

Because of this, Apple has adopted 9:41 in its advertising in more recent years. Even current promotional materials for the iPhone 7 on Apple's website right now all show 9:41 a.m.

So it's only fitting that Cook would wait until 9:41 a.m. on the dot to tweet about the iPhone anniversary. Jobs would have expected no less.