I am using org-invoice.el, which expects either CLOCKSUM or WORK properties to exist in an item; these properties contain some time duration record in HH:MM format.

I can't figure out how to generate those properties from a series of clock entries with any built-in user-facing functions, so I want to do it programmatically -- I can get the sum of the clock entries via (org-clock-sum-current-item), but how do I convert them to HH:MM format and insert it as a property?