I don't know what it is as a percentage, but globally there is in absolute terms significant amounts of East-West inter-timezone communication going on all of the time. Like literally, it does not stop happening and is happening right now.

Percentages are misleading when the population is several billions of people conversing and conducting commerce at near-light speed on a global scale. Just the three hour difference between the East and West coast of the United States is significant enough to be worthy of consideration on when to call people on the other coast (or just expect them to get back to you through asynchronous comms) because people on the West Coast are on average waking up three hours later as measured by GMT.

The same is true for the proposed global universal time. Without inter-timezone communication, everyone would just operate relative to their local solar noon, like they did pre-nineteenth century. The 'edge cases' of inter-timezone communication and travel are the only reason we need to care about any kind of coordinated time system.