Function queries with Json facet

[Mikhail Ibraheem]

Hi,I have a date field date_of_birth and have double field grade. I can easily 
get the sum of grades per date_of_birth value using:{        date_of_birth:{    
      type:terms,        field:"date_of_birth", facet: {   
"metric":"sum(grade)" }  }}
But what if I want to group only part of the date_of_birth? for example I want 
to get the sum of grade per year 2000?
This doesn't work:{        date_of_birth:{          type:terms,        
field:date_of_birth/YEAR, facet: {   "metric":"sum(grade)" }  }}
Please advise.
Thanks