Kate,
I had to tackle this with a recent webmap where I wanted to show only features that were projected to be replaced within 12 years. I was hoping to use a query filter on an Arcade field but that's not supported. What was suggested (and works) is to use the AGO Assistant tool to modify the underlying definition query used on the layer in your webmap. In my case, my layer's definition query was modified to this:
(a5facilitycat = 'Curb Ramp') AND (m9upgradeyr >= EXTRACT(YEAR FROM CURRENT_DATE()) ) AND (m9upgradeyr <= EXTRACT(YEAR FROM CURRENT_DATE()) + 12 )
My attribute field "m9upgradeyr" was field with the 4 digit year.
Hope this helps. Good luck!
Steve