One of the controversial claims of his last years was that of being able to build a land clock more accurate than any competing design. Specifically, he claimed to have designed a clock capable of keeping accurate to within one second over a span of 100 days. At the time, such publications as The London Review of English and Foreign Literature ridiculed Harrison for what was considered an outlandish claim. Harrison drew a design but never built such a clock himself, but in 1970 Martin Burgess, a Harrison expert and himself a clockmaker, studied the plans and endeavored to build the timepiece as drawn. He built two versions, dubbed Clock A and Clock B. Clock A became the Gurney Clock which was given to the city of Norwich in 1975, while Clock B lay unfinished in his workshop for decades until it was acquired in 2009 by Donald Saff. The completed Clock B was submitted to the National Maritime Museum in Greenwich for further study. It was found that Clock B could potentially meet Harrison's original claim, so the clock's design was carefully checked and adjusted. Finally, over a 100-day period from 6 January to 17 April 2015, Clock B was secured in a transparent case in the Royal Observatory and left to run untouched, apart from regular winding. Upon completion of the run, the clock was measured to have lost only 5/8 of a second, meaning Harrison's design was fundamentally sound. If we ignore the fact that this clock uses materials such as invar and duraluminium unavailable to Harrison, had it been built in 1762, the date of Harrison's testing of his H4, and run continuously since then without correction, it would now (April 2018) be slow by just 9 minutes and 45 seconds. Guinness World Records has declared the Martin Burgess' Clock B the "most accurate mechanical clock with a pendulum swinging in free air."