The Apple Watch App Store is open and ready for business, unless you want to sell a time-telling application.

An updated set of App Store Review Guidelines warns developers, without explanation, that "Watch Apps whose primary function is telling time will be rejected."

Eliminating third-party custom watch faces, the new rule (10.7) forces users to personalize their device using one of Apple's nine existing faces and fiddling with the display options.

"Choose a face and customize it however you like," the company website said. "Add more functionality. Switch colors and see even more detail, such as minutes, seconds, and milliseconds."

Apple tips "virtually endless" ways to make the smartwatch your own. What's more, you can constantly switch it up, sporting as many different looks over the course of a day as your imaginationand the Watch's pre-set featurescan muster.

Long-press the current watch face to call up the gallery; tap once to to choose a new one, or click "customize" to add or remove functions like weather, calendar, moon phases, and stocks. Check it out in the video below.

Straight out the box, you'll find nine faces: Chronograph, Color, Modular, Utility, Mickey Mouse, Simple, Motion, Solar, and Astronomy. But what if none of these catch your eye? Well, you're out of luck, according to Apple.

There is still hope for third-party faces, which could come along with Apple's announcement of a native SDKexpected later this year, according to 9to5Mac.

On the other hand, the tech giant may simply continue adding more of its own faces, which will boost the pool of options, but still deny outside developers a chance to flex their creative muscles.

In the meantime, the full Apple Watch store comes with categories like travel, health and fitness, communication, family, and work, offering applications that include Twitter, New York Times, Pacemaker, Instagram, and Yelp.

Not to mention Bandai Namco's Tamagotchi Classic.

The virtual pet, which first appeared in 1996 and returned in 2013 as a mobile app, is now available to download to your wrist. Watch wearers can take care of their pixelated friend from their wrist; send it to the restroom or feed it without digging for their phone.

If you're feeling nostalgic for the '90s children's toy, the Tamagotchi L.i.f.e. app is also available on select Android smartphones and tablets.

For more, see PCMag's review of the Apple Watch and the slideshow above.

Further Reading

Smartwatch Reviews