Deadlines are a blessing and a curse.
Having one means you've got a goal to work towards. Not having one means you don't have that anxiety cloud hanging over your head.
Are you avoiding setting yourself a deadline because you don't need one? Or do you not want a set point of failure?
If you don't have a deadline, the work can take the time it needs. But if you do and you don't meet it, does that mean you failed? It depends on what the work requires.
A missed deadline is bad for planes. They've got a tight schedule. International travel is important. But the creative work you're thinking about likely doesn't have as a strict a timeline.
Try it out. Say to yourself, 'I'm going to do this by next Monday, the 13th.' Then stick to it.
You'll be surprised what happens.
PS you can change the date to suit your needs but keep it specific. Someday isn’t specific.