no departure date... leave it empty. I did the same 6 months ago. You ae on a 90 day retirement visa. got to Siam Legal and pay the money to have everything done according to Thai law. I am getting my marriage visa on Friday for the 1 year extension. good luck in your retirement. they also assisted in getting bank acount with Kasiskorn bank.... no issues.
I went to Siam Legal and they will assist you with getting a Bank account. They go to the Kasikorn Bank office in their building. just under 6000 baht cost for their service.