I've been using an Apple Watch since mid-May. While I love my watch, one of the frustrating things about watchOS 1.0.1 is that it occasionally stops displaying the time of the next sunrise or sunset. Instead, it displays dashes.
Here's a process that has worked for me.
1. The first step is to make sure you have the right settings on the iPhone. Go to Settings > Privacy > Location Services.
2. Make sure Location Services is enabled.
3. Below that, make sure Apple Watch Faces can access your location "While Using."
4. Restart the iPhone by holding down the home and power button until you see the Apple logo. At this point, I usually recover the data. However....
5. If that doesn't reset the sunrise/sunset display, also restart the Apple Watch. Hold down the side button until you see the POWER OFF slider. Slide the POWER OFF icon to the right. To turn the Apple Watch back on, press and hold the side button. When you see the Apple logo, let go.
[UPDATE: Sep 2] SInce publication, I've noticed that, after a few days, the dashes return. On a sheer hunch, described on a recent TDO, I decided that it might help to have Apple's Weather app able to update in the background. iPhone: Settings > General > Background App Refresh > Weather (=ON). Since doing that, I haven't had any more problems.]
I'm hoping this occasional failure goes away in watchOS 2.0.