Thanks for the quick response. Sorry, I didn't explain in enough detail. The metadata for this project exists in an excel spreadsheet. There are 1800+ posts. I am importing the metadata (and creating the posts) into WordPress using csv importer. I have been assigning the actual date of the historical event to the "post_date" field in wordpress.
This import process works well, and all post 1970 dates are displaying well - but during the import process the older ones get changed to "1970". So the suggestion from others has been to put the historical date into another field, and sort the posts by this custom field.
However, this wont work for the timeline. So I was wondering if I could ask Simile somehow to look at a field other than "post_date" to pick up the date - and that way I could get around the problem. Is this possible.