date_diff_str function does not return expected results for 'month' part

The function date_diff_str() returns null when the part is 'month':
select date_diff_str('2015-12-22T20:02:48.407Z',clock_str(), 'month');
"$1": null

Other parts (e.g. 'hour') return correctly:
select date_diff_str('2015-12-22T20:02:48.407Z',clock_str(), 'hour');
"$1": 2168

Date diff processing may need to have a case for 'month':
https://github.com/couchbase/query/blob/master/expression/func_date.go#L1989
Also 'quarter'