cat /usr/share/backgrounds/gnome/adwaita-timed.xml
<background>
  <starttime>
    <year>2011</year>
    <month>11</month>
    <day>24</day>
    <hour>7</hour>
    <minute>00</minute>
    <second>00</second>
  </starttime>

<!-- This animation will start at 7 AM. -->

<!-- We start with sunrise at 7 AM. It will remain up for 1 hour. -->
<static>
<duration>3600.0</duration>
<file>/usr/share/backgrounds/gnome/adwaita-morning.jpg</file>
</static>

<!-- Sunrise starts to transition to day at 8 AM. The transition lasts for 5 hours, ending at 1 PM. -->
<transition type="overlay">
<duration>18000.0</duration>
<from>/usr/share/backgrounds/gnome/adwaita-morning.jpg</from>
<to>/usr/share/backgrounds/gnome/adwaita-day.jpg</to>
</transition>

<!-- It's 1 PM, we're showing the day image in full force now, for 5 hours ending at 6 PM. -->
<static>
<duration>18000.0</duration>
<file>/usr/share/backgrounds/gnome/adwaita-day.jpg</file>
</static>

<!-- It's 7 PM and it's going to start to get darker. This will transition for 6 hours up until midnight. -->
<transition type="overlay">
<duration>21600.0</duration>
<from>/usr/share/backgrounds/gnome/adwaita-day.jpg</from>
<to>/usr/share/backgrounds/gnome/adwaita-night.jpg</to>
</transition>

<!-- It's midnight. It'll stay dark for 5 hours up until 5 AM. -->
<static>
<duration>18000.0</duration>
<file>/usr/share/backgrounds/gnome/adwaita-night.jpg</file>
</static>

<!-- It's 5 AM. We'll start transitioning to sunrise for 2 hours up until 7 AM. -->
<transition type="overlay">
<duration>7200.0</duration>
<from>/usr/share/backgrounds/gnome/adwaita-night.jpg</from>
<to>/usr/share/backgrounds/gnome/adwaita-morning.jpg</to>
</transition>
</background>