In Episode 6 of Game of Thrones Season 7, "Beyond the Wall", Jon and the group that went beyond the Wall to bring back a wight to King's Landing find themselves surrounded by the dead. Just before this happened, Jon asked Gendry to travel to the Eastwatch-by-the-Sea and send ravens to Dany in Dragonstone to help them out beyond the Wall.

Even if the TV show is running the time a bit faster, episode wise, it would still take a bit of time for:

Gendry to travel back to the Wall. A raven to get from the Wall to Dragonstone. Dany riding off to beyond the Wall to help Jon and company. Given there's a lot of geographical distance at play here, how much time passed between Gendry splitting from the group and Dany getting to them?