According to many grammar textbooks, present perfect tense should not be used with specific time expression. Therefore, it is grammatically wrong to say "I have sold my house on 10 December 2018." However, I often see this type of sentence in the writings from the United States: In witness whereof, I have hereunto set my hand and affixed the Official Seal of said City this 26th day of August, 1954. Questions: Does the word 'this' make it an exception that 'present perfect tense' can be used with specific time expression in this construction? or Is the use of present perfect tense with specific time expression allowed only in American English?
The signer of a legal document is not narrating a past event, as in your example
I sold my house on 10 Dec. 2018.
but testifying that he/she has very recently signed and applied an official seal to the document on a specific date.
This usage is called the present perfect of the recent past. The whereof and hereunto are also clues that legal jargon is filled with 18th and 19th century forms which have no bearing on present day English.