Alison Barrow of Transworld in the UK is quoted in an article about Dan Brown in The Times Online, saying:

“There is never any clause from publisher to a novelist that they have to deliver at a certain time. We would not impose such a thing on a contract.”

Allowing that this is Dan Brown and that she probably is only referring to bestsellers like Brown, has anyone ever heard of a contract with a publisher that doesn’t specify a delivery date?