Using Nimbus of Light as the specific example, but the question is generic to *when* damage from emanations takes place.
For example. Let's say a Cleric's turn starts with her Nimbus already up, and two undead creatures are already within it. Then lets say she moves 30' on her turn, removing one of the original undead from the area but adding two additional undead into range. Do all 4 take damage since at some point in the round they were exposed, or does it have to be declared at a specific point in the round (e.g. start, end) when the damage takes place?