Is there an equivalent for "alter session set NLS_DATE_FORMAT = 'dd-mm-yyyy'" in Informix?

Hello everybody, does anybody know, if there exists an alternative for "alter session set NLS_DATE_FORMAT = 'dd-mm-yyyy'" in Informix? We found this statement in an application which uses an Oracle-DB. Now we have to modify this application so that a Informix-DB can be used. Unfortunately this statement causes a syntax error, because Informix doesn't know "alter session ...". To modity the environment variable DBDATE is not the solution we need, because this has no effect when the database is already open. This application offers the feature to modify the date-format within the running applikation. Best thanks Mathias