Start a new world then go to the button directly under where you name the worl and click on it until it says creative.
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You need the single player commands mod. Then you type /gamemode 1. This will change your survival world to creative. To put it back use /gamemode 0.
Build the map on Creative Mode then go to seedscope.net and upload your level.dat for the map on the Change Survival to Creative option. You should then download the new level.dat. Replace it with the old level.dat and when you start Minecraft, you should see that it is now a survival map.
If you mean Minecraft Classic, this is not possible. Minecraft classic is only meant for building, not survival. If you want to have survival, you'd have to buy the full version. To change day to night in Minecraft premium (full), as of 1.3 the command prompt was added to single-player so issue the command /time set night.
If you are the server owner (or can get the owners to change game mode or just yours) you can switch it to creative mode in the server conf file or by command in game by and op. otherwise a server in survival mode is staying in survival mode.
(this only works if you have singleplayer commands or if your a mod or up in a multiplayer server) /gamemode (name here) (1 for creative 0 for survival) ex: /gamemode blabla34 1 creative or to switch from survival to creative and back just do: /gm