I wondered this too! My French friend is asleep, but Google Translate says "it's dinner time" would be "c'est l'heure du dîner". I asked on Mastodon, where there are plenty of French-speakers, and some people said "c'est le dîner" to mean "it's dinner time" feels awkward, and some people said "c'est le dîner" does mean "dinner's ready" and is interchangeable with "c'est l'heure du dîner". So I've reported my answer ("it's dinner time") as "my answer should be accepted" and we'll see what happens, I guess!
In English the word "dinner" can mean either the meal eaten at lunchtime or the one eaten in the evening. However, historically it was class dependent. The lower classes (and especially the serving class) would have "dinner" around noon and "supper" in the evening, whereas the upper class would "lunch" at noon and have "dinner" in the evening.
The terms then get passed down through the generations and become somewhat disassociated from class and become a matter of custom and sometimes region.
In French, though, I believe that "dîner" refers to the evening meal. Someone please correct me if I'm wrong .