Concept of Older than x days and younger than y days

Hi Experts,

We are creating number of monitoring objects in SAP solution manager. I am having difficulty in understanding the concept of Older than x days and Younger than y days.

Can you give me an example? Basically I want to define a monitoring object on NAST output message from yesterday to today (the time at which the object runs in solution manager).

Thanks,
Suraj Pabbathi