I'm having a problem finding the mode over a subset of data...

I have a dataset describing retailers' prices of particular goods per
day, sorted by store, UPC, day. I am trying to find a way to record,
for each observation where the price is recorded, the modal price over
the previous and following 20 days.

so that for every observation with a price, modeprice=mode(price
variable from day-20 to day+20)

The only way I know how to compute the mode is with proc univariate,
but I'm not sure how to use it in this context.

There may be a simple solution to this problem: I am very much a SAS
beginner...