Its a little confusing because the request form asks for a date. But you can ignore that. Its valid with the same duration of your visa.
You can get a multiple re-entry permit giving you a unlimited amout of re-entries. How could you know all dates in advance? No a date is not required. I do this many years. Sometimes multiple but lately in covid times a single is sufficient for me.
I tend to do my yearly extention as soon as possible. In Chiang Mai that is 45 days in advance. My reason is you never know what might change. And as said the extention date still stays the same.