Using IEEE 1588 timestamps to measure packet arrival times?

[artag]

Light travels ~2m in 5ns.

Whilst I approve of the SI units, I find it convenient to remember that light (etc.)  travels about a foot per ns in a vacuum.
However , I'm old enough to be forgiven for remembering feet as a unit. I long ago ditched 186,000 mph for 3*10^8 m/s.