Is there a deadline for journal submission?
Generally speaking journals have multiple issues on some schedule from weekly to annually. A paper can be submitted at any time. The review process will then proceed and the paper may be revised on recommendation of reviewers. When the paper it ready for publication, an editor (managing editor) will schedule it for some future issue. 

So, there is no real deadline for normal journal submission. However, there is no guarantee that a paper submitted right now will be published soon. The review process takes time and there can be delays. But earlier submission (of a well written paper) is preferred. Submitting a sloppy paper early and hoping for early publication is probably a bad idea. The review will take longer and rejection is possible. 

However, some journals publish special issues, perhaps on a topic or to honor some person. For those, there is a deadline, but it is made clear when the "call" for papers goes out. 

For the question which is better - journals or conferences -, it depends on the field. In CS, for example, conferences are the preferred venue for new work. In other fields conferences are much less important. Your advisor will know which is best for you and also whether you should have any concerns about your progress.