Mobile opportunistic networks can enable self-organizing communications in complicated and dynamic scenarios. However, existing work overlooks the tempo-spatial properties of information dissemination to time-varying destination areas in mobile opportunistic networks, which reveal whether such networks can support emerging applications with dynamic dissemination objectives. In this paper, we investigate the tempo-spatial properties of information dissemination across a linear timevarying area boundary with the Brownian motion. Our investigations give the distribution of delivery delay under the one-copy and themultiple-copy forwarding policies, where the former has only one information carrier in the network and the latter has more than one information carrier at the beginning and each delivers a copy of the information to an encountered node with a certain probability. The analytical results demonstrate the power of the multiple-copy forwarding compared to the one-copy forwarding.