Oh, I'm sure that's what it is. What gets me is that the reservation was made with the month spelled out (as is the requirement at hilton.com), and it still shows as June 3-4. But somehow the hotel in Rome gets a date using the non-US convention of March 6 - April 6.
Surely this is something that Hilton should have no trouble dealing with, and I've never had a problem like this with all the reservations I've ever made (hilton or otherwise) outside the US.
Don't know if this is just a one-time glitch or what...