Sources with approximate publication dates
If you cannot establish the exact publication date for a source you can use c. for circa followed by an approximate publication date.
Smith (c. 1755) had several theories about human rights
Jones (c. mid-16th century) was critical of the monarchy
Lewis (c. 1400s) claimed that disease was caused by evil spirits.
Make sure you use the same approximate date in your full reference list as you do in the main body of your essay.