Informix products support DBTIME for compatibility with earlier products.
Informix recommends that you use the GL_DATETIME environment variable
for new applications.
The DBTIME environment variable specifies the end-user formats of values
in DATETIME columns for SQL API routines.
....
Tip: The DBTIME environment variable affects only certain DATETIME and
INTERVAL formatting routines in the ESQL/C function libraries. For information
about how these library functions are affected, refer to “DATETIME-Format
Functions” on page 6-16.