Did Apple originally plan to do this

Yes, Apple meant to do that. It started with iOS 7. If you look at the Calendar.app as well, you will also see the correct date.

Jailbreakers have had this since at least iOS 5 - if not earlier.

I think this started in iOS7. I would assume it was planned.

I never knew this.

Nope, apple did it by mistake. The clock "god" entered the phone and started moving the second hand.

nouveau-apple said:

I was just noticing that the icon of the clock app on the iPhone says the correct time but also has a functioning second hand.

Then I thought, if they could do that, how come other apps can't display information?

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Something like this does NOT happen by accident!

There seems to be few if any apps that do this so Apple is reserving this for themselves. I would bet there would be way more active icons if Apple would allow. Just think of the possibilities.

If Apple did allow such a thing our home screens would be crazy!

All advantages to jailbreaking.

I had live weather on my weather app under iOS 6. Of course, I swapped the icon for the one from Yahoo Weather and made the stock weather icon open Yahoo Weather, but live weather was there.

Apple may have reserved this to themselves but that certainly didn't stop jailbreakers from using it two or more versions of iOS before Apple chose to use it.

nouveau-apple said:

But why does the weather app not do anything?

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Because Apple chose to not animate it.

According to Apple and the weather icon it's always partly sunny in my area haha

People have long since been talking about app icons that actually give real time information.

For whatever reason Apple doesn't seem to feel this is necessary. I would be all for it but, like everything iOS, it's up to Apple.