Dating the action in The Chronicles of Barsetshire

The chronicles of Barsetshire were written in 1855, 1857, 1858, 1861, 1864 and 1867 yet it seems that the action was not always set in the year of writing. In The Warden, Eleanor Harding is 24 but in The Last Chronicle of Barset she is a little over 40, yet only 12 years had passed between the publication of these two books. Also, Lily Dale is 19 when jilted by Crosbie, but in The Last Chronicle of Barset she is said to be 24 but that this event that marked her life had taken place 3 or 4 years previously! When quoting letters between the characters Trollope does not give the reader the exact date, choosing instead a formula such as "May 186-" for example ( a technique which greatly annoys me I must admit). Does anyone know of anything mentioned in the books which could be used to date the action?