Natural sundial

Clever people these Royal Parks managers.

You see, their dilemma will have been "...what to we do with that young oak that died at the tail end of last year, you know, the one that didn't quite make it through the frosts and snowfall that kept us busy throughout December!?"
"Cut it down and there'll be public outcry!"
"But it's dead!"
"We'll have to trim it down though!"
Fred in the corner pipes up "...who cares, as long as we can still have our 'elevensies', it can stay there for all I care, I'm not volunteering to cut out that stump!"
"...but Fred," says his partner Burt, "if we break your watch whilst we're cutting it down, how are we going to know when we can have our cuppa?"
"....something will give us an indication." says Fred hopefully.