calendar travel time in both directions

So the feature allowing the addition of travel time to an event is great. But in order to be able to use calendar for all my scheduling needs, I really need to be able to do the converse, that is add in the travel time from an event (either back to the origin or to the next appointment).

Is there any way to add this? I'd settle for a simple reflection of the 'To' time to start with.

The auto calculation feature isn't going to help because I need to retroactively update a year's worth of calendar entries. It is based on your current location, among other difficulties.