erri_wan said:

However I also wonder if it's due to the excessive creativity of Italian dubbers, Warner Bros over-zealousness or (also possibly) a mistake by the Italian dubbers in saying centuries instead of years (that would set it in late 23rd Century).

The films tagline when marketed in '71 was, "THE FUTURE IS COMING", "THX 1138 - A love story filmed on location in the 21st century" and "THE FUTURE IS HERE IN, THX 1138 - A love story filmed on location in the 21st century" it was on the original posters and trailer etc, that doesn't necessarily have to mean the setting was in the 21st Century but I find it interesting. The script doesn't mention anything about it as far as I can remember, haven't read the novel though.