Temporal search took a big step forward yesterday with the announcement from Andy Hertzfeld of the launch of News Timeline on Google Labs. This is exactly the kind of thing I'm trying to build in Flash so it's exciting to see Google's take on it.
Historical data can be displayed for dates as far back as the 1400's and from what I can tell they are using the proleptic Gregorian calendar for any dates prior to the Gregorian reform in 1582. Most of the early content is pulled from Wikipedia but dates closer to the present return much richer data. The temporal granularity can be set to day, week, month, year or decade.