A DATE type stores an integer number of days since December 31, 1899 in four bytes which will handle dates for 2^31 days which is over 5.8 million years into the future.
A DATETIME type can store anything from milliseconds to years in a size that's appropriate to the precision.
To customize this DATETIME format,
choose your locale appropriately or
set the GL_DATETIME or DBTIME
environment variable. For more
information about these environment
variables, see the IBM Informix: GLS
User's Guide.
The values of DBDATE and DBCENTURY affect the number of digits in the year.
DBDATE defaults to MDY4/ (for the US English locale) which includes a four-digit year.
DBCENTURY controls how to interpret data that includes two-digit years.
See also GL_DATE and DBTIME.
Edit:
A DATE type stores an integer number of days since December 31, 1899 in four bytes which will handle dates for 2^31 days which is over 5.8 million years into the future.
A DATETIME type can store anything from milliseconds to years in a size that's appropriate to the precision.
See Chronological Data Types which says, in part: