To access the app's camera, go to any day in your calendar that you want to add a second to, tap to add a snippet, and swipe right.
Open a day's screen, tap to add a snippet and click on the camera icon at the top right of the media picker.
You can even launch directly to the app's camera from our notifications if you enable this setting with the iOS app.
FYI: The second will be applied to the current day and the video will be saved to your camera roll. Access the whole video in your Photos app if you'd like.