We consider a system of real-valued spins interacting with each other through a meanfield Hamiltonian that depends on the empirical magnetisation of the spins. The system is subjected to a stochastic dynamics where the spins perform independent Brownian motions. Using large deviation theory we show that there exists an explicitly computable crossover time t c ∈ [0, ∞] from Gibbs to non-Gibbs. We give examples of immediate loss of Gibbsianness (t c = 0), short-time conservation and large-time loss of Gibbsianness (t c ∈ (0, ∞)), and preservation of Gibbsianness (t c = ∞). Depending on the potential, the system can be Gibbs or non-Gibbs at the crossover time t = t c .MSC 2010. 60F10, 60K35, 82C22, 82C27.