To get 10,000 years -- the max interval value -- I can use year or month:
SELECT INTERVAL 10000 year, INTERVAL 10000*12 month
What is the value that I would use for day to get the 10,000 year value? Any variation I've tried of it either does not convert-up from day to year or overflows (if I go beyond). Is there a way to convert day into year in the output, or BQ never converts it?
As an example:

