Please consider merging branch timezone of repo https://git.spwhitton.name/propellor.
Adds Timezone.configured.
I think that this works fine on stretch, but on Jessie there is some oddness. For example, if you set the timezone of a host to US/Arizona, the apt reconfiguration will put America/Phoenix in /etc/timezone, resulting in the property reporting a change every time that it is run. I think this is harmless.
--spwhitton
I generally consider properties that do work every time to be a minor bug.
I wonder if it would be better to preseed tzdata rather than writing the config file. I observe the same substitution from eg, US/Eastern to America/New_York in the file when reconfiguring noninteractively, but reconfiguring interactively I can select US/Eastern and that gets into the file.
Anyway, merged as this is certianly a good starting point.