The button at the far right of the toolbar lets you set alarms.
Alarms can be set relative to any daily sunlight event at any location. They can be set for a specific day or to repeat indefinitely. They can repeat on certain days of the week, or all of them.
Tapping this button brings up the Alarms panel. Tap the + symbol at the top right to add a new alarm or reminder.
Set the date you want to start with (default is today); then pick which daylight event you want an alarm for, e.g. "Sunset"; then pick the offset, e.g. "10 minutes before".
The location will default to your current location, or you can also set an alarm for a different city by tapping on the "Location" indicator, if you want to call your sister in London at sunset, for example. If you are traveling, you can have your alarm times change as you move around the planet by setting "Travel Mode" to on!*
The options screen lets you pick one of our custom sounds for your alarm (or you can use the iOS default sound), what days (if any) it should repeat, and an optional label to remind you why you set that alarm. "Persistent Alarm" should only be used when you don't want to miss the alarm — to wake you up in the morning for instance — it will go off every minute until you resolve it by opening the app. Make sure your device's sound is loud and the mute switch is not engaged or you will not hear the alarm.
To delete an alarm, swipe across it in the alarm list and click the Delete button. You can also delete a non-repeating alarm by setting it to occur in the past.