Hm. Thanks for bringing this up!
Something must have changed, or I made a stupid mistake. I also get PT0.219S (ISO duration, meaning 0.219 seconds) where it should be 219 days.
At least
<% moment("12-24", "MM-DD").diff(moment(), "days") %>
shows 219 (the correct # days). Which should be enough for the intended “days til Xmas”.
If, for some reason, it needs to be more exact, one could probably use
<% moment.duration(moment("12-24", "MM-DD").diff(moment())).as('days') %>
to arrive at 219.278469375 (at this moment in time).
Sorry if I messed something up!