International Call Timing App?

I am going through a list of 105 colleagues who live all over the world (Phillipines, Korea, South America etc).

My task is to group them into groups of a dozen by time zones to provide a mix of various countries in each group but not make anyone have to call in too late or too early. I'm using excel, looking up each time zone by UTC + or - XX:XX, an color coding the country cells to do it manually. Do you happen to know if there is a free software program or excel trick to make this any easier?