File retention if I'm getting it right?

Hi, I want to delete some of my files automatically after some period of time, but I don’t know if I’m getting this right based on retention app description. Let’s say that I’ve created a file on January 1st on my hdd, but I’ve send it through client app on windows on January 15th. And I’ve set this file with this tag to be deleted after 30 days. So what is creation day according to nextcloud? Is it 1st or 15th when using desktop client?