As prissi stated, the server cannot pause. There is no provision in the networking code to allow commands to be processed without time progressing. When a client performs an action, it sends a request to the server. The server marks a time in the future to execute the action, and broadcasts that to all connected clients. Once that time is reached, all clients (and the server) actually perform the action. Without time passing, the future action time will not be reached. Such a feature could be added, but there's really been no call for it...
For jointly preparing a map, you could set the map's starting year before the intended actual start year; And/Or set a high bits_per_month and just let time progress while preparing. Loading the game in singleplayer mode you can then run/fastforward/skip time until the desired start.