Genitive Help with Time

Question

I came across this sentence, "Уточни, пожалуйста: какого числа будем разговаривать и в какое время!" Why is the genitive being used for какого числа?

Answer 1

I tend to see an abbreviation here: (днём/в день) какого числа? It's something like 'on the day of which date?' Then, the missing Russian day technically 'belongs' to that date (чей, чего -> genitive case).