Quantiles of a subset of data

Excuse the request from an R novice! I have a data frame (DATA) that has
two numeric columns (YEAR and DAY) and 4000 rows. For each YEAR I need to
determine the 10% and 90% quantiles of DAY. I'm sure this is easy enough,
but I am a new to this. 

quantile(DATA$DAY,c(0.1,0.9)) 

10% 90% 
 12  29 

But this is for the entire 4000 rows, when I need it to be for each YEAR.
Is there no way to use a "by" argument in the quantile function? 

Thanks for any help you can provide. 
David