Peter Erskine's "Spectrum of Time"

"Spectrum of Time" is a permanent rainbow sundial calendar installation... Hour and month lines painted on the walls and floor of the 40' X 40' X 40' industrial museum space mark the hours, summer and winter solstices, and the spring and autumn equinoxes with astronomical accuracy. A 30' X 30' cross of solar spectrum light powered by the rotation and tilt of the earth tells the time and date. On cloudy days a laser pointer driven by a solar tracking program fills in for the rainbow.

from here.