Is there a symbol used with AD and BC to mean "possible date"?
Circa:
(written abbreviation c); (ca)
(used especially with years) approximately:
He was born circa 1600.
I don't know whether historians use it in this way, but one of the many uses of the tilde (~) is "approximation".
Robinson discovered the island in ~202 BC.
would therefore be read/understood as
Robinson discovered the island in approximately 202 BC.
The Oxford English Dictionary prepends the letter c to indicate an approximate year:
c1400 (▶?c1380) Pearl
a quotation from a manuscript of around (= circa) 1400 preserving a text probably composed around 1380. (The symbol ▶ preceding a date indicates that this is a date of composition, not a manuscript date.)
This is also used in the Middle English Dictionary, for example.
If historians are uncertain about the date, is there a simple and short notation they can add to the date, to indicate the uncertainty? I'm looking for something like this, but more professional:
Robinson discovered the island in 202 BC (?).
Yes, there is a symbol and it is (Ahem......) (?).
Your use of (?) is professional and correct.
According to Sussex University Website:
The question mark also has one minor use: it may be inserted into the middle of something, inside parentheses, to show that something is uncertain.
The question marks on the poet's birth and death dates indicate that those dates are not certain.Summary of Question Marks:
- Use a question mark at the end of a direct question.
- Do not use a question mark at the end of an indirect question.
- Use an internal question mark to show that something is uncertain.
[University of Sussex]
Also from White Smoke:
Use a question mark enclosed in parenthesis (?) if a date or another number is unknown or doubtful among the experts. Do not use a question mark to communicate that you are unsure of the information you write, as this weakens the authority of your writing.
Joan of Arc, 1412 (?) – 1431, is considered a French heroine.
Also see Jozef Pelsudski Institute.