I just put in miesac and was rejected for miesacem, what am I missing here? The translation in flag shows miesac.
"miesiąc" and "miesiącem".
Well, I guess that was according to grammar. Either "Czerwiec to miesiąc lata" or "Czerwiec jest miesiącem lata".
Isn't it possible to use jest?
Yes - 'Czerwiec jest miesiącem lata'.
I've reported it. Maybe they'll fix it soon.
So "lata" is the genitive of "lato", as well as "roku" is the genitive of "rok"?
Not quite; "lato" is not a word in this context.
Take a look at https://en.wiktionary.org/wiki/rok#Polish and 'show' the declension to see the various forms of these two words.