Upvote:1
You might try at one of CBP's deferred inspection sites. More information is available at their page on the subject. There is one such site in Maryland, at BWI airport.
If you are a US citizen, however, you are probably out of luck, because not stamping your passport is not an error, and, as the page notes
The Deferred Inspection Sites will only correct errors made at the time of entry.
If you are not a US citizen, you will probably be given an entry stamp with the proper date, but you will probably need some sort of evidence to support your assertion that you entered the US on that date.