Question: Up to how far off can the Deal Timeball be from Coordinated Universal Time?
Related documents: [0]  [DOC] Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London. GMT was formerly used as the international civil time standard, now superseded in that function by Coordinated Universal Time (UTC). Today GMT is considered equivalent to UTC for UK civil purposes (but this is not formalised) and for navigation is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); these two meanings can differ by up to 0.9 s. Consequently, the term GMT should not be used for precise purposes. [END] 
[1]  [DOC] Deal Timeball is a Victorian maritime Greenwich Mean Time signal located on the roof of a waterfront four-storey tower in the coastal town of Deal, in Kent, England. It was established in 1855 by the Astronomer Royal George Biddell Airy in collaboration with Charles V. Walker, superintendent of telegraphs for the South Eastern Railway Company. It was built by the Lambeth firm of engineers Maudslay and Field. The time ball, which, like the Greenwich time ball, fell at 1 pm precisely, was triggered by an electric signal directly from the Royal Observatory. [END] 
Answer: 
0.9 s.
0.9 s.