First--my subject title was the excuse for making the post; I figured it would be a new word to at least some people--and also would give a solving hint.

I'll white out what the book says, so if anyone still wants to wrangle with the puzzles, skip this:
My Day Closed Is In Immortality = MDCIII = 1603, = the death of Queen Elizabeth the first.

LorD haVe MerCI Vpon Vs = L+D+V+M+C+I+V+V = 1666, the Great Fire of London. Yes, it was addition! Sometimes I wonder where my brain is---the pluses are right there in the book!

Addison denounced chronograms as 'the results of monkish ignorance' and sneered that 'tricks in writing require much time and little capacity'. (p. 60)