This is no diary, folks. It's a memoir. It's Pip recalling his whole life's story at once. By our calculations, Pip the narrator is about 57 when he tells this story—which means that we're always in the position of knowing just a little more than Pip-the-boy does. Let's look at just one passage from the very beginning:
At such a time I found out for certain that this bleak place overgrown with nettles was the churchyard; and that Philip Pirrip, late of this parish, and also Georgiana wife of the above, were dead and buried; and that Alexander, Bartholomew, Abraham, Tobias, and Roger, infant children of the aforesaid, were also dead and buried; and that the dark flat wilderness beyond the churchyard, intersected with dikes and mounds and gates, with scattered cattle feeding on it, was the marshes; and that the low leaden line beyond was the river; and that the distant savage lair from which the wind was rushing was the sea; and that the small bundle of shivers growing afraid of it all and beginning to cry, was Pip. (1.3)
What we love about this is that Pip manages to both tell us the story from his grown-up, all-knowing adult perspective (talking about himself in the third person), but also convey the real feeling of being a terrified, shivering 6-year-old. Hey, it's not a classic for nothing.