Translation:They are going to arrive around eleven.
But no way to write it
Duolingo translation: "They are going to arrive about eleven." A bad one...
They are going to arrive on about the eleventh! In English!
Las once is 11 o'clock, not the eleventh of the month.
Use either "on the eleventh," "about the eleventh," or "around the eleventh." Don't, however, combine the two.