Some spells have implications "when the spell is cast again", such as mage hand or imprison no longer applying to the previous target. A wizard character is arguing that the simulacrum he's casting disappears at the beginning of the casting time, while the cleric in the party is arguing that the simulacrum remains until the end of the 12 hour casting time.

Personally, I think the wording of "is cast" implies that the casting is complete, but I want to make sure there's nothing official I'm forgetting before I get all grammar lawyer on my players.