Quote Originally Posted by klopp View Post
да, да завтра я буду в универе. Во сколько ты на обед? как раз встретимся.
“Yes, yes, I will be at the university tomorrow. What time will you go for lunch? Just then we'll meet.”

Quote Originally Posted by klopp View Post
*which grammar rules is this sentence made of?
There is an ellipsis in this sentence. «Во сколько ты (пойдёшь) на обед?». The word «пойдёшь» is omitted. This is perfectly correct for colloquial language.

When using directional prepositions, verbs of motion can be omitted in colloquial speech.
E.g. you can say:
«Я на обед» instead of «Я иду на обед» “I go for a lunch”;
«Я только до магазина и обратно» instead of «Я только дойду до магазина и приду обратно» “I'll just reach the shop and come back”;
and so on…