Earth measurements were the standard units of measure used in the United Federation of Planets. It is possible that the UFP did not actually use Earth measurements, but that their usage on screen is an example of "implicit translation" (such as the characters in the film K-19: The Widowmaker speaking English when the viewer understands that they are "speaking" Russian.) This is implied by scenes such as the one where Kirk remarks to Abraham Lincoln that the Enterprise could convert to minutes. (TOS: "The Savage Curtain" )