Another small issue: Leading zeroes are removed from days, as expected, when using an inline bibliography (first example) but they are *not* removed when using an external .bib file (second example):

$ cat date-leading-zeroes-month-day.md
@item1

---
references:
- title: Title
  id: item1
  issued:
    day: 02
    month: 01
    year: 1999
  author:
    family: Author
  type: article-journal
...

$ pandoc -S --filter pandoc-citeproc -t markdown-citations date-leading-zeroes-month-day.md
Author (1999)

<div class="references">

Author. 1999. “Title” (January 2).

</div>


$ cat date-leading-zeroes-month-day.bib

@article{item1,
    Author = {Author},
    Date = {1999-01-02},
    Title = {Title},
}

$ echo [@item1] | pandoc -S -t markdown-citations --bibliography date-leading-zeroes-month-day.bib
(Author 1999)

<div class="references">

Author. 1999. “Title” (January 02).

</div>