I'd like to be able to display a clock that would at least show the server time. Preferably also would be able to show the local time (you could switch between these 2, no need to show both at the same time).
It wouldn't have to have an impact on the data being transferred over the internet all the time. Make the clock ask server time once at startup. Then remember the difference with local time, and you can then know server time on the client by adding/subtracting from local time for as long as you are logged on. So the impact on network traffic would be near to zero. I think this would even be possible without asking the server time at all, as long as the client knows in which timezone I am (if my computer is configured correctly) and at what timezone the server is. But do take summer/wintertime changes in account, which are not the same everywhere in the world.

For me, this would be enough and very helpful to know when actual server restarts will take place, or other events that are announced either by SSG or for example by guild messages (for running raids etc.) This clock may be pretty small.

But I could imagine that some people might also like a more complex control (this can be arranged for example by appearances, like the focus orb also has 3 different appearances) Then this could also for example display a date, day of week or maybe even a complete calendar. Also it could remind people of ongoing events like the DDO bonus days (which bonus is currently active) or the bigger events like Mimic hunt or other festivals.

Like I said, I would already be happy with a small clock indicating the time on the server. It doesn't even have to be visible all the time, as long as I can just check it every now and then.