Google Calendar is the same basic calendar app that you can find on Nexus Android phones, and it doesn't offer anything special over the built-in HTC calendar app. Still, it's available if you need more Holo in your life.
What you can't match
Although your HTC One now resembles the version Google sells, you're still at the mercy of your carrier when it comes to receiving system updates. If a new version of Android is released for the Google Play edition of the HTC One, it could be weeks (or more likely, months) before your phone receives the same features and bug fixes. While that delay leaves you vulnerable to bugs or security holes, it's the compromise you make when you buy a subsidized phone from a carrier.
But it's not all bad news: Your HTC One benefits from HTC's custom camera and imaging software, which allow it to take better-looking photos than you'd get from the stock Android camera software. Sure you're missing out on the ability to capture 360-degree panoramic Photo Spheres, but you can use the Zoe camera to create animated GIFs—arguably a much more useful feature.
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