So, I have finally made use of the Schema.org tags, and they work very well!
I did notice one issue about date coding. It seem that dates that don't include a month or day and have 0 in those fields get interpreted as “the day before the 1st” or “the month before the first month”. So a date with only a year, such as 1895, might show up as 1894-11-29, or a month and a day before Jan 1st, 1895.
It's not a big deal for my current use, and I don't think it's worth trying to fix this if limitations of schema.org make it non-trivial to fix, but I did want to point it out in case others might want to use the embeded microdata.
I know it took me a while to get around to using this, but thanks again for adding this!