How would they know when I mined the coins before I sent them there to be sold about 3 days later?

Another question is, how can I figure it out?

Back when software mining was profitable and difficulty levels were high 2 or low 3 digits, I just let the thing free run for months as a "nice"-d process. So I'm not even sure when I mined them, other than "a long time ago" and then got rid of them late last summer/fall.