“But for building excitement I think it would be best to send it after it got dark (the timing of which you could determine from the date and location).”
On a somewhat related note, I recently went hunting for phone apps to help me better sync up with times as determined by the sun, rather than numbers on a clock. I like the feeling of getting enough sleep and of being up before the Sun, and I'd like to be relatively unaffected by jumps to and from daylight savings time.
I learned to think of a day in terms of...
Astronomical Twilight: The Sun is between 18 and 12 degrees below the horizon. Dark according to everybody except astronomers.
Nautical Twilight: The Sun is between 12 and 6 degrees below the horizon. You can easily distinguish the sky from the ground, so sailors can use the horizon to navigate.
Civil Twilight: The Sun is between 6 and 0 degrees below the horizon. Light enough to play outside without artificial lighting.
Sunrise/Sunset: The instant the upper edge of the sun appears or disappears on the horizon.
Golden Hour: The Sun is between 0 and 10-12 degrees above the horizon. No exact definition, but photographers consider this a time for great lighting conditions.
Solar Noon/Midnight: The Sun is at its highest/lowest point above/below the horizon.
I was interested in easily visualizing these times, and in setting alarms based on these times (possibly with fixed offsets).
I decided Nautical Dawn makes a good snooze alarm if you want to get a head start on the day, and Civil Dawn is a good backup if you didn't feel like getting up at Nautical Dawn. It helps psychologically that these times are attached to real events, and that the sky is starting to lighten at Civil Dawn.
I bought and tried a bunch of iPhone and Android apps. Below are my favorites.
iOS
Sol: Displays all the above times on a moving wheel, and can set alarms by them.
Android
Nothing I liked as much as Sol on iOS, but the widget capability allows an immediacy and combination possibilities that iPhone doesn't.
Sundroid: Widget shows pairs of times and can set alarms.
Daytime: Widget lists civl dawn/dusk, sunrise/sunset, and a prominent message such as "3:29 to sunset".
LunaSolCal: Lists all the times mentioned above (except golden hours) in a compact widget
AstroClock: Cool widget visualization with three concentric wheels: day/night, then lunar phases, then seasons
Sorry for the length of this, but I just dove into this a few days ago and I'm happy with what I ended up with, so I thought I'd share.