Background color is slightly offset for some reason

[sandebert]

(I tried producing this image using the examples of this project but failed to reach them because of #64. So I used the example from vis.js instead, I hope that's ok.)

It seems like the background color is slightly offset for some reason. This is taken from the Background areas example

My first thought was that the areas were defined using a time that wasn't midnight, but looking at the code that doesn't seem to be correct. My best guess is that my timezone is playing a role here. I'm at UTC+2, which seems to sort of match the visual offset.