[O] Convert from date to week number, howto?

[François Pinard]

Hi, Orger friends :-)

I sometimes have to convert from a date to a week number, for when I am
shift-arrowing a clock table summarizing weeks.  It was easy at the
beginning of the year, but I see it requires more thought as year
advance.  I can write little programs to do so, but I wonder if someone
has an easy trick in Org mode (or in Emacs)?

If none, maybe that C-!, or something related to dates or calendar
display, could show the week number for each week?  (random ideas! ...)

François