http://www.sunsurveyor.com/ I'm just going to leave this here.
Also, any of the surveying tools used for solar panel placement are great for garden planning. With these tools you can estimate total solar exposure over the course of the year for any location in your garden. And - you don't have to wait for a particular time of day.
Let me recommend a couple of Android apps, instead of a watch. I assume they or something similar are available for iOS if you use iPhone.
SunSurveyor and Sundroid, which mentions civil, nautical, and astronomical twilight times among its features.
nice! i like to use sun surveyor for most position and path finding.
using that pv watts site, I get a winter irradiance estimate of about 30% of summer, and a similar system ratio. that seems more accurate!