I've found three dates so far in Goldsmith ,so don’t believe everything the editor of the Oxford World Classics tells us :-)

JA includes the date of 6th May as being the date of Anne Boleyn's letter to Henry VIII, omitting to give the year, as you correctly say, Emmeline :-)

So why did she do this?

I think she was directly lampooning Goldsmith here. He is very admiring of Anne Boleyn.( probably because she was, and still is, in some circles, regarded as a martyr to the Protestant cause, having openly supported the Protestant religion during her lifetime.).

Let’s look at Goldsmith’s text:

from Letter XXXI:

Upon such slight suspicions was this unhappy queen sent to the Tower, in order to wait the execution of her sentence. She who had been once the envied object of royal favour, was now going to give a new instance of the capriciousness of fortune: she ws ever of a cheerful disposition, and her easy levities, perhaps, disgusted the gloomy tyrant. She has distributed in the last year of her life, not less then fifteen thousand pounds among the poor, and was at once their protector and darling. Upon being conducted to her prison, she sat down to address the king, by letter, for mercy; in this she insisted upon her innocence in the strongest terms: You have raised me, she said, from privacy to make me a lady; from a lady you made me a queen : and from a queen I shall shortly become a saint. On the morning of her execution, she sent for Mr Kingston, the keeper of the Tower, to whom, upon entering the prison she said Mr Kingston, I hear I am not to die till noon, and I am sorry for it , for I thought to be dead before this time and free from a life of pain. . The keeper attempted to comfort her, by assuring her that the pain would be very little, she replied I have heard the executioner is very expert and (clasping her neck with her hands, laughing) I have but a little neck.. Kingston, who gives this account, continues to observe, that he had seen many men and women executed, but never one whose fortitude was equal to hers. She was beheaded soon after, May 19, behaving with the utmost decency and resolution.

Letter XXXI.

So- JA twists this: she gives us the date of the letter and not the date of Anne’s execution. But , in imitation of Goldsmith, she omits the year…..