Might depend on where you are getting the date from. It was quite common for public buildings in the 19th Century to have a foundation stone (libraries, chapels, hospitals etc). Although not actually part of the foundations, the date on it would be a year or more before completion. Conversely, the houses on my estate were started in 1999, but the estate went on for a couple of years. However, all the ones built after Dec '99 have a little '2000' plaque on them...even the ones built in 2001!