Filemaker's Date fields (a) require a full date (year, month, day) and (b) are limited to a range starting at Jan 1, 0001.
Based on your example, I'd suggest using two Number fields: one for the exact or starting year, the other for the end year (empty in case the year is known exactly).
That could be a real challenge!
Hmm, if the dates are all in terms of centuries you might make things work by having 2 number fields, Century1, Century2. Put the earliest century in Century1 and make BC centuries Negative.
Structure finds like this:
To find all records whose date range includes 1st Century BC,
Enter find mode
Put < -1 in Century1
Pug > -1 in Century2
Perform the find
Thanks you both for the quick replies.
I'm going to test your suggestions, I think the solution could be a mix of them.