"The girls are a little sad."
Translation:Las chicas están un poco tristes.
The posts are marked "1 month ago", which seems to be Duolingo's relevant form of timekeeping.
Because that isn't the convention.
If I understand it correctly and when both verbs are the equivalent of the English "to be", "ser" usually indicates permanence while "estar" indicates "at the moment".
To wit: "Yo estoy en California" (because although I've lived here for 35 years, I could be in Arizona in a couple of hours). But "Yo soy de Florida" (because no matter where I roam, I will always be from Florida).
A lot of these usages become matters of convention; so even if I somehow know I will remain in California forever (let's say I'm locked up for life without parole), the verb for current location remains "estar".
IIRC, it's the same with adjectives. Some have come to use estar or ser according to custom and we just have to memorize them (which is why it is GREAT that DL now gives us so many more drills).