are both methods used?

Oracle uses its own internal format to store dates.
Date data is stored in fixed-length fields of seven bytes each,
corresponding to century, year, month, day, hour, minute, and second.

http://download-west.oracle.com/docs...4dat.htm#41259


Julian dates allow continuous dating by the number of days
from a common reference. (The reference is 01-01-4712 years
BCE, so current dates are somewhere in the 2.4 million range.)
A Julian date is nominally a noninteger, the fractional part
being a portion of a day. Oracle uses a simplified approach
that results in integer values. Julian dates can be calculated
and interpreted differently. The calculation method used by
Oracle results in a seven-digit number (for dates most often
used), such as 2449086 for 08-APR-93.

http://download-west.oracle.com/docs...3datyp.htm#796