What about the time difference between illusion and outside world?

[[ For one, even if Itachi had actually done it, he would not have stayed for very long. 

Secondly, it’s understood their meetings are private, so Orochimaru can be alone in their designated meeting room for quite some time– around the time it would’ve taken them to do so. Itachi can falsify other sensations within genjutsu, like touch, sense of smell, etc. 

Tsukuyomi has also been proven to be able to control motor functions (such as forcing Kabuto to make hand-signs to release the Edo Tensei), so Itachi can have Orochimaru “freshen up” and then end the genjutsu at a natural time point, where Orochimaru is none the wiser. 

Lastly, I don’t think Orochimaru’s going to run outside to see how much of the sun has passed after their encounters.

I had initially thought of actually writing it happening, since Itachi is willing to do quite a bit to ensure Sasuke’s safety, up to and including giving his body up, but that’s too much of a risk for him, and why does he have one of the most powerful Sharingan abilities if he is not going to put them to use? ]]