Biblatex: How to convert date field into year and month field?

This is something Biber can do in its tool mode.

Unfortunately, the solution originally suggested here ceased to work a while ago. Following https://github.com/plk/biber/issues/301 Biber was adjusted so things would work again. The fix will be available in Biber 2.15. For versions in between, I'm afraid I can not offer a simple Biber-based solution. (Maybe bibtool can help here.)

Biber version 2.15 and above

We need the following config file, called, say, biber-date.conf

<?xml version="1.0" encoding="UTF-8"?>
<config>
  <output_fieldcase>lower</output_fieldcase>
  <datamodel>
    <fields>
      <field fieldtype="field" datatype="literal">year</field>
      <field fieldtype="field" datatype="literal">month</field>
    </fields>
    <entryfields>
      <field>year</field>
      <field>month</field>
    </entryfields>
  </datamodel>
  <sourcemap>
    <maps datatype="bibtex">
      <map map_overwrite="1">
        <map_step map_field_source="date" map_match="[0-9]{4}-([0-9]{2})" map_final="1"/>
        <map_step map_field_set="montha" map_field_value="$1"/>
      </map>
      <map map_overwrite="1">
        <map_step map_field_source="date" map_match="([0-9]{4})" map_final="1"/>
        <map_step map_field_set="yeara" map_field_value="$1"/>
      </map>
    </maps>
  </sourcemap>
</config>

To convert the date fields in your .bib file run

biber --tool --configfile=biber-date.conf <yourbibfile>

For the example file augustus.bib

@article{kattan_timeseries_2015,
  title = {Time-series event-based prediction: An unsupervised learning framework based on genetic programming},  
  journaltitle = {Information Sciences},
  author = {Kattan, Ahmed and Fatima, Shaheen and Arif, Muhammad},
  date = {2015-04}
}
@book{lorem,
  title = {Lorem},  
  author = {Anne Uthor},
  date = {2015}
}
@book{ipsum,
  title = {Ipsum},  
  author = {Anne Uthor},
  date = {2015-08-07},
}

the output (which you can find in augustus_bibertools.bib after the biber --tool --configfile=biber-date.conf augustus.bib run) is

@article{kattan_timeseries_2015,
  author = {Kattan, Ahmed and Fatima, Shaheen and Arif, Muhammad},
  journaltitle = {Information Sciences},
  month = {4},
  title = {Time-series event-based prediction: An unsupervised learning framework based on genetic programming},
  year = {2015},
}

@book{lorem,
  author = {Uthor, Anne},
  title = {Lorem},
  year = {2015},
}

@book{ipsum,
  author = {Uthor, Anne},
  date = {2015-08-07},
  title = {Ipsum},
}

---

Old versions of Biber

We need the following config file, called say biber-date.conf

<?xml version="1.0" encoding="UTF-8"?>
<config>
  <output_fieldcase>lower</output_fieldcase>
  <sourcemap>
    <maps datatype="bibtex" map_overwrite="1">
      <map map_overwrite="1">
        <map_step map_field_source="date" map_match="[0-9]{4}?-([0-9]{2}?)" map_final="1"/>
        <map_step map_field_set="month" map_field_value="$1"/>
      </map>
      <map map_overwrite="1">
        <map_step map_field_source="date" map_match="([0-9]{4}?)" map_final="1"/>
        <map_step map_field_set="year" map_field_value="$1"/>
      </map>
    </maps>
  </sourcemap>
</config>

The regular expressions are not particularly elegant, but the first one copies the month position of the date field to the month field, the second maps the year position to the year field.

Now you just need to run biber --tool --configfile=biber-date.conf <yourbibfile>

On the example file augustus.bib

@article{kattan_timeseries_2015,
  title = {Time-series event-based prediction: An unsupervised learning framework based on genetic programming},  
  journaltitle = {Information Sciences},
  author = {Kattan, Ahmed and Fatima, Shaheen and Arif, Muhammad},
  date = {2015-04}
}
@book{lorem,
  title = {Lorem},  
  author = {Anne Uthor},
  date = {2015}
}
@book{ipsum,
  title = {Ipsum},  
  author = {Anne Uthor},
  date = {2015-08-07},
}

the output (which you can find in augustus_bibertools.bib after the biber --tool --configfile=biber-date.conf augustus.bib run) is

@article{kattan_timeseries_2015,
  author       = {Kattan, Ahmed and Fatima, Shaheen and Arif, Muhammad},
  date         = {2015-04},
  journaltitle = {Information Sciences},
  month        = {04},
  title        = {Time-series event-based prediction: An unsupervised learning framework based on genetic programming},
  year         = {2015},
}

@book{lorem,
  author = {Anne Uthor},
  date   = {2015},
  title  = {Lorem},
  year   = {2015},
}

@book{ipsum,
  author = {Anne Uthor},
  date   = {2015-08-07},
  month  = {08},
  title  = {Ipsum},
  year   = {2015},
}