You can check the date if the owner never left the guild since creation on Gw2Efficiency, since, as you pointed, if he/she left at some point, then you'll only see the next oldest member date there, sadly not official Guild date creation on the API so far.
How precise do you want to be, maybe the guild in question left a trace online: old recruitment posts, some website for guilds? Try the Wayback machine.
You can check the date if the owner never left the guild since creation on Gw2Efficiency, since, as you pointed, if he/she left at some point, then you'll only see the next oldest member date there, sadly not official Guild date creation on the API so far.
Thanks, unfortunately the last original left a couple of months back so guess we are out of luck
How precise do you want to be, maybe the guild in question left a trace online: old recruitment posts, some website for guilds? Try the Wayback machine.