Why does the author of "Edward Jenner and the History of Smallpox and Vaccination” most likely include dates when discussing Jenner’s work to replace variolation?

to show that time was important to Jenner in relation to his work
to show the many years it took for the vaccination to replace variolation
to show why Jenner was ridiculed for his work on the vaccination program
to show how the British Parliament slowly came to accept the value of the vaccination