The sun dial was donated by Henry Bolckow in 1879 (aong with the park itself in 1868)
It was designed by John Smith of Stockton and gives the times in New York and Melbourne as well as Middlesbrough.
The plaque below it shows corrections for the time of year, accounting for BST its pretty much spot on for the time I took the photo.
there is a similar sundial in peoples park in halifax west yorkshire also designed by mr smith,well worth looking at.