When the runway at Farnborough was being re-surfaced, I realised it would change from '25' to '24' within 12 months, so I made some enquiries and found they calculate rates of change in 4 year 'epochs' rather than just as they show it on half mil charts, so it was actually due to change within a couple of months, thus we were able to paint the new surface with 06/24 rather than have to re-paint it within 12 months.