Omit `day` attribute in `<date>` boilerplate element for RFCs

[str4d]

What went wrong?

The RFC Editor has informed me that:

We publish RFCs with only month and year, with the exception of April 1 RFCs.

mmark renders the document date in the title boilerplate here: https://github.com/mmarkdown/mmark/blob/3944c1c00c36aa11468fcc21f32410f920be3463/render/xml/title.go#L177-L196 (at least, I think it's here; but that renderer hard-codes "January" as the month, and I can't find anywhere else that renders title dates).

This renderer does allow for the day attribute to be omitted (if it is zero); however, there is no syntax for encoding such a date into the Markdown source.

mmark should omit the day attribute from the <date> boilerplate element (for non-April-1 RFCs), and maybe also permit a date item in the title block that doesn't include a day.

What version of mmark are you using?

2.2.39

What is the most minimal markdown snippets that shows the problem?

%%%
    title = "Foobar"
    submissionType = "IRTF"
    date = 2023-11-01

    [seriesInfo]
    name = "Internet-Draft"
    value = "foobar"
    stream = "IRTF"
    status = "informational"
%%%

renders:

<?xml version="1.0" encoding="utf-8"?>
<!-- name="GENERATOR" content="github.com/mmarkdown/mmark Mmark Markdown Processor - mmark.miek.nl" -->
<rfc version="3" ipr="trust200902" docName="foobar" submissionType="IRTF" category="info" xml:lang="en" xmlns:xi="http://www.w3.org/2001/XInclude" indexInclude="true">

<front>
<title>Foobar</title><seriesInfo value="foobar" stream="IRTF" status="informational" name="Internet-Draft"></seriesInfo>
<date year="2023" month="November" day="1"></date>
<area>Internet</area>
<workgroup></workgroup>

</front>

</rfc>

If I try to explicitly set the day to zero (2023-11-00), I get the following error (and similarly if I leave off the day and use 2023-11):

Failure parsing title block: toml: line 4 (last key "date"): Invalid TOML Datetime: "2023-11-00".
Empty 'value' in [seriesInfo], resulting XML may fail to parse.
Empty 'stream' in [seriesInfo], resulting XML may fail to parse.
Empty 'status' in [seriesInfo], resulting XML may fail to parse.
Empty 'name' in [seriesInfo], resulting XML may fail to parse.

If I try to set the date field to "November 2023", I get the following error:

Failure parsing title block: parsing time "November 2023" as "2006-01-02T15:04:05Z07:00": cannot parse "November 2023" as "2006"

[miekg]

toml's date construct requires a full date which an I-D does need that, but an RFC doesn't.

think it's fine for the RFC editor to remove this attribute from the XML and there should be no workarounds from our side

[miekg]

maybe I can do something here

[str4d]

toml's date construct requires a full date which an I-D does need that, but an RFC doesn't.

Yep, I have confirmation from the RFC Editor that the day attribute indeed should be present for an I-D, but not for an RFC.

216 LGTM, thanks!