CUPERTINO—Apple revealed details about watchOS 4, its newest update to the Apple Watch's operating system, back at June's WWDC keynote. Now we know when the update will be pushed to the public: September 19. This news comes on the heels of Apple announcing the Apple Watch Series 3 model with standalone LTE service at the company's event today.

WatchOS 4 will change the look and feel of the Apple Watch slightly with aesthetic updates, a new vertical app dock, and a few redesigned apps. The Music app has an entirely new look in watchOS 4, the Workout app has a new color scheme and "tap and go" button options, and the News app can now be used to receive headlines on your wrist and save articles for later reading.

WatchOS 4 will also bring the new Siri watch face to the device, which proactively gives you information on your wrist based on where you are, the time of day, upcoming calendar events, and more. The Apple Watch already has Siri capabilities, but Siri's AI and machine learning technology is used in this watch face to provide more personalized information to those who use it. Other new watch faces include a "kaleidoscope" design and designs for Toy Story characters Woody, Jessie, and Buzz.

One of the most practical uses of the Apple Watch is activity tracking, so it's no surprise that watchOS 4 continues to expand on the device's fitness chops. Now there will be more focus on movement and exercise motivation, with features including prompts to move, "inspiration" cards that tell you how much more exercise you need to do to reach your daily goal, monthly activity challenges based on your exercise history, and new celebration animations when you achieve a goal.

Apple also improved the Workout app and trackable exercises. There's a new high-intensity interval training workout option for those specific routines, and the pool swimming workout has been improved to use the accelerometer's motion data to know which type of stroke you're doing and when you're resting at the edge of the pool. Those of us who do multiple exercises in one workout routine will enjoy the new multi-exercise feature, which lets you end one exercise and immediately start another.

Apple Watches running watchOS 4 will make use of GymKit as well, which lets the device talk to exercise equipment. Let's say you're running on a compatible treadmill: you can wirelessly pair your Apple Watch with the treadmill so they exchange fitness information. Your Apple Watch knows more about your body than a treadmill does, with tons of logged information including height and weight, but the treadmill is considered more accurate in estimating your total distance. The two machines can exchange information so your Apple Watch will log your distance more accurately, and it can use that along with your body's information to estimate caloric burn. Gym machines may also be able to display heart rate information from the Apple Watch on their displays so you don't have to glance down at your wrist every time you want to check.

This technology needs to be built into gym machines, which means manufacturers like Life Fitness will need to update existing machines and create new ones. The trickle-down will take time, so you may not see an Apple Watch-compatible machine in your local gym immediately. However, we do know Apple has been working with gyms and equipment manufacturers to push out updated machines across the country.

While not a medically certified device, Apple is also launching a heart rate study that uses the watch's heart rate sensor to detect atrial fibrillation, irregular heartbeats that can be a precursor to stroke.