mergedate option is what you are looking for. See The
You might like
mergedate=false strictly separates the date specification from the
date label. The year will always be printed twice.
All dates are printed twice.
mergedate=minimum merges the dates whenever the full date and the
date label are exactly the same string. If the date is a bare year
number and there is no
extrayear field, the date specification will
Knuth 1948's date is not printed twice here.
mergedate=basic [...] will always omit the date specification if the date is a bare year number.
Knuth 1986a and Knuth 1986b's date are not printed twice either.
If you prefer the date to go into the parentheses at the front, you can use the
datelabel option. The default value is
year, so you only get to see the year. You can choose between
iso8601 (see pp. 50-51 of the
The latter options print the full date.
This option is probably best used with the default