It seems to do fine with halb vier on its own.
And when I enter Wir treffen uns um halb vier. and then click on the (incorrect) translation, I get two options to pick from, and the other one seemed correct to me.
So it may be the new(ish) full-sentence translator thingy that still has some problems which the older (more phrase-by-phrase) one didn't have.
Yep: I think you're spot on (*): it's an intriguing beast! The correct alternates are sitting there in the background on all such time constructions: hopefully they'll begin to push through to the front in the near future.
(*) Start with "halb vier" or "um halb vier", which are, as you noted, mercifully correct, and then throw something (anything!) to the left or right of those expressions: the incorrect rendering is instantly triggered.
I'm not sure what you mean with detecting a difference -- usually, only one of the cases will make sense since the verb determines whether it takes an accusative object, a dative object, or both.
So if you hear wir waschen uns die Hände, then uns must be dative while if you hear wir sehen uns im Spiegel, then uns must be accusative.
And for verbs that can take one or the other (e.g. glauben: accusative with fact, dative with person) generally only one makes sense: wir glauben uns nicht will have dative uns because wir is a person and not a fact.
If you mean -- when you see a new reflexive verb, how do you know whether it takes a dative object or an accusative one?
Good dictionaries should make this clear, and show that it's, for example, sich (Dat.) etwas leisten but sich (Akk.) etwas trauen. (Or they may just mark sich (Dat.) as sich (Akk.) is the default.)
There is a helpful discussion on another message thread from 3 yrs ago. Hope this helps:
BrendanMcKinley: is there a difference between "Es lohnt sich" and "es wert"? thanks in advance
sakasiru: Yes. "lohnen" means you get some kind of profit from whatever you do. "wert sein" just means it is worth the effort in some way. Rescuing a kitten might not "sich lohnen" (and it sounds very cynical to evaluate that), but it is definitely "wert" ;)
az_p MOD: Is sich lohnen explicitly saying that it is financially worth it? Otherwise, I don't get the distinction from wert sein.
sakasiru: No, not necessaryly financially. But you have some benefit from it, while "wert sein" can just mean some moral or emotional value. It can also mean that something has the exact price for its value, while "lohnen" means you make a good bargain in the deal, because you pay less than it's worth.
Well, literally it would be "We will meet at half past three", but that doesn't convey the same sense of incompleteness that the German has when you leave out the object.
It feels a bit like "I will wait for in the dining room" or "I will pour the drink into at half past three" -- those scream "something is missing here!" in a similar way to your German sentence.
(At least if treffen is meant as "meet". It would be acceptable if it meant "hit [a target with a projectile]", as in "We've been doing target practice all day already and have been missing the target the whole time, but at half past three, we will hit." In this case, the English sounds odd without an object -- "we will hit the target" sounds better to me -- but the German sounds fine to me without one in this meaning.)