Found this one today en route to spending the afternoon with my 
sister...I managed to persuade VHF1 that this really was "on the way" 
('though I'm not sure what someone would have made of that statement 

)
A
 fascinating puzzle which I spent a fair amount of time working out - 
tackling from both ends to correct for any possible error (which after 
all is important !) then made a real "schoolboy" error by mixing up two 
of the figures from the dates which put me close, but not close enough 
(certainly for this !) and the ever patient Amberel was good enough to 
point this out and then save me from a second error which would have put
 me on the wrong side 

The
 location of the cache was absolutely unmistakable at GZ, thanks to a 
very precise hint, so after retrieving and signing, I stood for a few 
minutes in the glorious sunshine just taking in the significance of 
where I was standing.
Many thanks to Amberel for an absolutely fantastic theme for this cache and for the mental workout too 

Thanks and Merry Christmas Amberel...