Scan Engines are designed to find charts with a specific set of technical criteria on a specific date. Occasionally, we get a question from someone who is trying to use the scan engine to find stocks with a specific set of technical criteria over a range of dates. We call this the "within" problem since they are looking for something that happened "within" a certain time period. For example, "Show me all the stocks that had a MACD crossover within the past month."
The reason our scan engine doesn't support these "within" scans is because you cannot use them in a real-world trading environment. From a high-level perspective, the purpose of scanning is to develop scans that can help you decide which stocks to buy or sell "soon" - i.e. before the data used in the scan changes significantly. The standard scenario is to run your scans after the market closes in preparation for placing orders early the next day. While some of the results from a "within" scan may still be valid, others results may have become invalid by the time the scan is run and, what's worse, you cannot easily tell which is which.
We strongly recommend refining a "within" scan so that it refers instead to "today". For example, take the scan above and turn it into "Show me all the stocks that had a MACD crossover today." You can then use the "Starting" field (at the top of our scan interface pages) to see the results of the scan on any previous day you choose.