Time scale

[i-and]

1. The comments on the Time type do not specify the time scale used. Maybe it should be clarified explicitly?
   2 Is UTC used for all timestamps in ROS2?
   3 If UTC is used, how is the situation correctly worked out when a corrective second occurs and in a situation when some nodes know about the fact of its occurrence and some do not? There will also be a non-linearity of the clock running.
   4 Has the option of using an International Atomic Time (TAI) timeline been considered for timestamps (PTP also uses TAI)?