I regularly do have to work with and also have friends in other time zones. Despite doing so for the better part of two decades and not being bad at mental maths most of the calculations involved, especially with DST at different start and end dates, are a total headache. It would be much easier to have a list of "person x is available from global time y to z" data and check where that overlaps. Not to mention all the issues around "meeting at the same time every week" when "the same time" has no meaning between two time zones with different DST.