I have a paper that explains how to do exactly that. It's called "Demystifying Intervals", and tries to work through the common misperceptions of SAS intervals, which are pretty powerful once you get the hang of it. The custom interval scenario is on pages 6 and 7, the code that creates the custom interval in the example is on pages 13 and 14.

https://www.lexjansen.com/mwsug/2017/BB/MWSUG-2017-BB042.pdf