That 10 cent stamp was issued in 1907, leaving little doubt that the '08' bit means 1908 (which, I'll guess, is probably all you need for your charity listing anyway).
If you compare your stamp with this one, you'll see that the righthand Roman numerals from your example don't appear in the one that I've linked to here:
https://i.ebayimg.com/images/g/xaMAAOSw42Nf1M0U/s-l1600.jpg
I read the postmark on my example as 22nd August 1908 and thus those on yours as 16th April 1908. My guess is that the 'XI', which is on the end of yours, indicates 11am (possibly at a busy post office, with multiple collections each day), whereas the absence of a time in my example might indicate that it was posted at a rural post office, with just a single daily collection (and thus no need to indicate a time).