Trips in the past when home city has changed?

I recently joined dopplr and added most of the trips I did while living in Berlin. I now moved somewhere else and will do my trips from there, but I wanted to add some other trips that I did while living in London over a year ago.
First of all, if I add a trip in the past (before I joined dopplr), will it count the carbon from my current city? or from the city where I lived when I joined? (Berlin)
In both cases, is there a way I can enter another city as the home base back then?

It seems quite a few people who move have problems with the home city switch, would it not be easier to just be able to enter where one lived between specific dates?