Okay, in researching this, it seems that the date literal format that Informix recognizes is affected by Informix options like DBDATE. See this page for example. That article suggests that DBDATE can be set on the client side, perhaps in the ODBC driver setup. However, it looks like all of the formats Informix likes have separators between the date parts.
Here is what I would suggest as a workaround:
Set up your filter the way you did originally:
Then, use the red triangle menu in the filter to change the filter type to "Custom Expression":
That will change the filter to just be an entry field:
Then, try changing '20130216' (or whatever your date is) to '2013-02-16', and then click outside of the Custom Expression box to let it process. See if you still get the error. If so, try '2013/02/16'. Failing that, you may need to ask the adminitrators of your Informix database how your installation of Informix is expecting date literals to be formatted (a.k.a., what value does DBDATE have). Also, I don't have the Informix ODBC driver installed at the moment, but you could look there and see if there is a way to configure DBDATE in the driver such that the default date format that Query Builder is using would work for you.
HTH,
Eric