How are blanks indicated for placeholders in Chinese (like ???)
Generally putting XX is fine unless formal. People use that a lot orally. X is usually pronounced as 叉, but can vary based on region.

Formally and also very commonly for missing name is using 某. 

王某 (someone with surname 王 and one-character given name)

王某某 (someone with surname 王 and two-character given name)

某某/某某某 (very general, someone with unknown name, no implication on length of name)

For dates, if exact year is unknown or of no importance, you say 19世纪 (19th century), 19世纪80年代 (1880s)