A chronogram is an inscription where some of the letters, when interpreted as numerals, can form a date. The practice was begun during the late Roman Empire and can be found on items as diverse as gravestones, maps, and housing. A natural chronogram has all the numerals in the correct order and a pure chronogram has exactly one numeral for each word in the phrase.
My Day Closed Is In Immortality
In this example the highlighted letters spell out the date MDCCIII, commemorating the death of Queen Elizabeth I of England in 1603. In the image below, a portrait of Lord Coleraine from the British Museum collection, the inscription around the image has been interpreted to give a date of 1703.
