Phys.Org has an article about a new experiment with ions that achieves higher precision in measuring time dilation. They accelerated ions to 40% of C (light speed) and measured the time dilation to an unprecedented level of precision. The new measurement reaches a precision of a few part per billion. The article is at Phys.Org

Bill Gill

C is not the speed of light in a vacuum.
C is the universal speed limit.