overlap - Change CSS property (like background colour) for overlapping DIVs -

i have calendar-style app displays events colour-coded divs of varying lengths absolutely positioned on timeline. these events must not overlap. want able highlight overlaps via css. example, non-overlapping events display green background, if 1 gets scheduled overlap other (completely or partially) want both change to, say, orange. there css way or must examine events conflicts? (i know how latter, seems there should css solution.)