Web App Idea: Preference-Based Approach to Coordinating Across Time Zones

My daughter is going on an international trip, and needed to coordinate a time to Skype with people in multiple time zones. By multiple, we're not talking U.S Easter, Central and Pacific. Or even a European time zone. We are talking, U.S., Europe, Australia. There is going to be zero overlap of normal working hours.

Obviously the ideal solution involves a web-app to coordinate reasonable times. Reasonable here being defined as anytime during a given party's normal waking hours.

I found a number of websites that do this to an extent. This one seems as good as any. The 4-color scheme requires some mental math, but then I realized, instead of trying to track the three colors that represented waking time, I just needed to eliminate the 1 color (red) that represents sleeping time.

The enhancement I would like to see is personalization. Instead of default typical hours, I would like the ability for each of the participants to enter times that conform to their personal schedule.

For extra credit, there could be two tiers: preferred, and can-make-it-work. Results would be returned, in sort order, using a two-stage algorithm. First stage would maximize the number of participants who can attend. E.g., if a time slot meets 100% of can-work but 0% of preferred, it will make the cut, whereas 90% preferred and 0% can-work won't make the cut. Then within first cut, sorting would be a points system. Say 2 points for each preferred and 1 point for each can-work.