Adapt period terminus model to earliest/latest year format
Zero-pad year values according to XMLSchema#gyear format

http://www.w3.org/TR/xmlschema11-2/#nt-yrFrag
Spell "millennium" correctly
Handle trailing spaces
Correctly handle leading "C.E."

Before, the c. was (understandably) getting eaten up as "circa"
Handle BP dates that include a C14 designation