Is there a legal time period to begin campaigning?

Probably a dumb question but you have to start somewhere. I have tried to find information on this and just haven’t been able to so I figured this was a good place to ask. So, with the huge uphill battle that third parties face in the United States, is there a law that prevents them from beginning to campaign sooner. Say, right now, today, is there anything that would stop a third party from naming a candidate for the 2028 election?