How do I set up a world that is only daytime, on the same server as one with a normal day/night cycle?
Solution 1:
Don't use your main server for tests, even a small one
Using other worlds on your main server for tests is a really bad idea. Firstly, it does not give you a fully independent test/building environment (many values, like time, are changed globally, server resets are done globally, etc.). Secondly, if you break something on a test/building server, it doesn't damage the main one, while a test world might make the damage also touch your main environment.
Thirdly, a separate test server allows you to give full creative mode permissions to all the players, while it is rather dangerous on your main one, even if you think that you can really trust everyone who plays with you. Yes, your permission management plugins can malfunction, and I have seen Bad Things Happening because of that.
Once you have learned to set up one dedicated server, setting up another one is exactly Ctrl+C, Ctrl+V, and some changes in the server.properties file (Port, server name).
World Edit is a plugin that allows you to easily move structures from one place to another after you set it up.
Whenever I need to test something in Minecraft, I simply set up a testing server, then stop and/or delete it when it is no longer needed.
Of course, you can only use this solution if your hardware can handle two servers at the same time.
My answer is true even for a small server for a couple of friends, but is especially needed on any server that hosts more than, like, 7 people. Playing Creative and playing Survival are basically two different games, and it is better if they are played on two different servers that borrow each others' results when needed.
Having two distinct servers also solves a problem of moving stuff from one place to another. Players just log on to the server they choose at the moment, and all you need to move a structure on your main server is copy and paste a schematic via WorldEdit. Don't forget to check the amount of pricy blocks they use if you have an economy set up, for example, someone might try to sneak in a bunch of diamond blocks.