045 Subfield $c - Formatted pre-9999 B.C. time period:

Subfield $c contains a formatted time period consisting of as many numeric characters as are needed to represent the number of years B.C. First indicator position values 0-2 specify whether the time period in subfield $c is a single date, multiple dates, or a range of dates. Each pre-9999 B.C. time period associated with the heading is contained in a separate subfield $c.

Example:

650 #0$aGeology, Stratigraphic$yMesozoic.

045 2#$c225000000$c70000000