UDC Connect: Problem with timestamp field for selection

We are loading data from an Informix table with a timestamp field in it. The timestamp field is splitted in the DataSource into two fields, one for the date and one for the time. Now would like to make a selection on the timestamp field to select only the records from the last 24 hours (as an example). The problem is now that due to the splitting of timestamp field into two fields we are not able to define a correct selection. Especially when the selection should span more then one day and also considering the time.

Has everybody faced this problem before?

Thanks for your help