I have a sea time question. Please help

Hello all, I currently hold a DDE Unlimited HP. I want to add a Chief of uninspected fishing vessel unlimited HP. Apparently I need about 200 more days sea time to get my raise in grade. My question is… Can I submit a sea time letter from a company I worked for before the dates of sea time I submitted for my first license about 15 years ago? Does that make sense? I can probably track down sea time from previous employers of 20 years ago or so. That would probably add over 1000 days to my sea time. Hopefully they are still in the industry.
Which brings me to another question I guess. What if a company is no longer around? Can you get a sea time letter from a previous captain? What if I was a captain? Any help would be greatly appreciated.