"Arriviamo a casa alle diciotto."
Well I am familiar with a military clock how ever it wouldn't be common in English to say "let's meet at 1800 hours" anyway.. As in English we would just say see you at 6 and the assumption is made depending on the time in the day really even without saying '6 in the evening' . In Italian how ever, they are more familiar with a 24 hour clock, hence the use of 'diciotto'
oh right, in London no one would say 1800. With that in mind no one would say 18 either, I'm more referring to when writing, if someone said in Italian , let's meet at 18 I would hear it in a different context I suppose, like not translating literally just getting the context from it