This could be the graph of 2008:

But as it happens it's not share prices, it's ΔT ("Delta T"), 1620AD - 2000AD.

Remember that because the Earth's orbit is squashed, a day can be 30 seconds longer or shorter than 24 hours. The exact amount changes over time, but it's nicely predictable.

However there's also an unpredictable "secular" change, partly caused by the Earth spinning slower and slower (due to friction with the moon via the tides), and partly caused by the Earth having a wonky axis, arising from changes in landforms and the "lava" in the core sloshing around.

The size of these secular change is tiny. One day might be longer or shorter than 24 hours by just a thousandth of a second. However, taken over two years, that daily increase of 0.001 seconds can add up to 1 second. And for satnav and astronomers, that's huge; hence, leap seconds are introduced, as will happen on Wednesday night.

ΔT, then, is essentially the cumulative number of leap seconds over the centuries.

To put it another way: if the Doctor were to lose a perfect watch in pre-Roman Britain, and found it in 2008, then it would be out by 3 hours. And that's what ΔT and leaps seconds are: they're the difference between a perfect watch and the realities of the wonky Earth.