Dante-
Yes, it was fairly straightforward. Open up AGO Assistant and then navigate to your webmap of interest and chose "View an items JSON" from the "I Want to..." menu. The second "block" of JSON code details with the layers in the webmap. Scroll through it until you find the layer you need to tweak. Once you find the layer, you'll want to zero in on the DefinitionExpression property. Here's what it looks like in my edited webmap:
The later half of the definitionExpression sets the criteria for a date between the current year and 12 years from now. The above expression is based on an actual date field but you could also do the same thing with a text based field as well. Originally, the data I was provided had the 4 digit year stored as text. Here's the same definition but for a text based field:
"((M8UpgradeBy = 'Transition Plan') OR (M8UpgradeBy = 'Capital Project') OR (M8UpgradeBy = 'Overlay Program')) AND (m9upgradeyr >= EXTRACT(YEAR FROM CURRENT_DATE()) ) AND (m9upgradeyr <= EXTRACT(YEAR FROM CURRENT_DATE()) + 12 )"
While the version based on the text year field worked for display purposes, I had to change the underlying data to be a date field because the date slider widget I was using in WebApp Builder wasn't honoring the query expression. Once I changed it to hit an actual date field, all was well.
Steve