help about datetime

We are working in informix version 4 under unix, but with little previous training. Also our english level is good enough to understand the manuals, but the appendix that explains datetime, interval, etc. looks obscure to us. Could somebody help us with datetime and interval variables, and the extend function ?. For instance, if we subtract two columns or memory variables defined as DATETIME YEAR TO MONTH, how can we get an INTERVAL result of the type yy-mm and how can we use the result, if we need INTEGER variables containing the year and the month part ?. We would apreciate examples, a commented piece of code, etc. Thank you in advance.