I wish I could help. There's a custom datepicker in NewsPublisher, but that's for the front end. I've never tried implementing one in the Manager.
My only advice would be to look at source of the Manager page in Chrome Dev. Tools or Firefox and see how it's done. You can't see it with "View Source" because almost everything is created on the fly with JavaScript. Once the page is loaded, though, it's all visible in Dev. Tools and Firefox.
I don't know how to modify that for use in another field. Maybe someone else can help.
kjf,
Did you ever solve this one. By adjusting the ids to be unique (from copy and pasted DOB markup) I was able to get date field to visually show but not functional.
I assume there is a bind to the js that is not happening.
"One of these days I will get around to my own website... Its only been about 12 years... maybe tomorrow

"
Thanks for sharing. If when I get this figured out, I will post back. I imagine it all needs to be handled in js and not in hardcoded html
"One of these days I will get around to my own website... Its only been about 12 years... maybe tomorrow

"