time after time

My two smallest have reached 2 years 5 months. They have just had a big developmental leap surrounding the perceptions of time.
For small people their understanding of time is often limited to right now. We ask them to wait and they meltdown because to them, anything beyond now is sort of equivalent to never. You may try and prompt them to tell about something exciting that happened yesterday or last week and they look at you blankly.
I was buckling one of them into her carseat the other day and she told me a story of something that had happened to her last time she was being put in her carseat. We got home from playcentre and Aunty was regaled with a story of the friend they had been playing with that morning. And suddenly, just like that they have a past! They have the ability and the tools to see a specific event and describe it without it having to be seen and experienced in the present.
Simultaneously I am noticing that the future is more tangible too. Whereas before asking them to wait for a glass of milk while I finished what I was doing must have felt like a flat refusal suddenly they are able to understand that even if it doesn't happen this instant, it will still happen. After many months of them wanting everything all at once and right now, they can understand 'finish this first, then you can have that'. That's one less tantrum in my day!
I'd love to know exactly what's happening inside that makes this new way of thinking possible.