An example would be 'I'll let you know what train I'll be getting a bit nearer the time' - you've made an arrangement to meet someone sometime in the future, and you'll let them know when you will be arriving shortly before the day.
BE = British English. It's not an expression that I'm familiar with, that's why I asked. A quick Google search turned up similar contexts. To convey your message, you could say something like "Je vous dirai quel train je prendrai quelques jours d'avance" (a few days beforehand) or "dès que je le sache" (as soon as I know). Perhaps a francophone can suggest a more idiomatic equivalent that more closely captures the meaning "closer to the date of the event in question."