It has taken me a very long time to find an envelope for the following behavior.

I have some charts that are being generated programmatically and added to an
existing file. Basically I create a chart, some data, and a few styles, and
stick them into the document tree at a reasonable point. This works fine until I
start trying to apply time/date formatting to values on my X axis.

I find that I'm able to use a style like this:
<number:date-style style:name="N2"><number:month
number:style="long"/><number:text>/</number:text><number:day
number:style="long"/><number:text>/</number:text><number:year
number:style="long"/></number:date-style>

But if replace the second '/' with, say, '-':
<number:date-style style:name="N2"><number:month
number:style="long"/><number:text>/</number:text><number:day
number:style="long"/><number:text>-</number:text><number:year
number:style="long"/></number:date-style>

then when my document opens and the chart is rendered, the dates show up as
floating point numbers with no formatting.

I can use the chart editor to change the format for the X axis to what I should
be seeing and then save the document. OO writes it out in the canonical form,
separating the chart out into an embedded object. The automatic style is exactly
the same:

<number:date-style style:name="N116">
<number:month number:style="long"/>
<number:text>/</number:text>
<number:day number:style="long"/>
<number:text>-</number:text>
<number:year number:style="long"/>

And when I open the document, the formatting works just fine.

It appears to me that in the circumstances I have, OO only accepts a small
number of formats and rejects everything else, despite the XML being perfectly
reasonable ODF and being the same as what OO writes out and parses correctly in
its canonical format. HH:MM:SS works, HH:MM works, MM/DD/YYYY works; any
variation seems to be ignored. (More may work -- I don't know. I just figured
this out.)

What's going on here? Have I missed something? Is there a workaround that
doesn't involve creating embedded objects? (That's not necessarily a big deal
for me, but it is more work.) Why does this not work as it seems it should?