The year filter, like others indicates a number of document instances linked to it in brackets.
Eg on dev.gawati.org default dataset as "2015 (2)".
If you actuvate filter 2015, you get more than 2 documents.
This may or may not be linked to multiple metadata fields being of date format, and all of them being considered in the filter. However if so, the same rule should apply for generating the indication in brackets (2).
The year filter, like others indicates a number of document instances linked to it in brackets. Eg on dev.gawati.org default dataset as "2015 (2)". If you actuvate filter 2015, you get more than 2 documents. This may or may not be linked to multiple metadata fields being of date format, and all of them being considered in the filter. However if so, the same rule should apply for generating the indication in brackets (2).