As has been said - at school you had dinner money, there were dinner ladies which means the meal at dinner time is dinner!!
After school / work you go home and have your tea at teatime.......it isn't difficult.
I'm from an estate and we have dinner in the middle of the day.
My partner and her parents are a little more upper class than me and they call their evening meal "dinner"
We've had countless discussions on this over the years and I'm bored with it now, so the other weekend when they invited us round for "dinner" I made a point of going round there at midday, knowing full well they meant the evening.
Was round there for around 30 minutes and asked if I had the day wrong as I thought they had invited me round for dinner but I couldn't smell anything being cooked.
I think their response included the word smartarse!!