Converting dayTimeDuration to yearMonthDuration

Hello people.

I'm sorry to bother you with questions like this. I just couldn't seem
to find an answer anywhere else. I'm currently using XSLT 2.0 and
taking advantage of XPath 2.0. I'm trying to get the age of someone
based of:

(current-date() - xsd:date(birthDate))

which returns a dayTimeDuration. The issue is I'd really like to use
years-from-duration but that expects a yearMonthDuration. I tried
casting from dayTimeDuration to yearMonthDuration but to no avail.
Does anyone know a way to get a year result from a dayTimeDuration?

Thanks,

Gary