Perhaps there should just be a text box allowing users to enter in their offset as a number of minutes (or mathematical expression: "+10*3600+1800")

"how long ago" or color coding doesn't sound all that usefull to me. "how long ago" because things get cached and only an absolute time will ever REALLY tell you something. color coding simply because it seems like it would get distracting. (but hey: as long as there's a check box to turn it off, who am I to complain)