Hiro: June 15th.
Sae: November 3rd. How are you going to celebrate it?
I think only the four "original" girls have birthdays because the dates were made up by someone working on the first season of the anime. Ume seems to be okay with it though since she's drawn some art on their birthdays, but I don't think she's going to give dates for any of the other characters.
https://anidb.net/perl-bin/animedb.pl?show=anime&aid=4743AniDB lists them on their character pages.