Everyone I know seems to say that using a sand timer (hourglass) on shabbos is forbidden. The question is why? Is it because of sifting? Is there some other melacha involved? Is it actually forbidden?
The issue at hand is measuring. See the Shulchan Aruch siman 308 siff 51 and the Ramma who says the accepted practice is to not use sand timers.
See also Rabbi Ribiat's 39 Melachos volume 4 page 979. In the note section he quotes the Shulchan Aruch and says to see Shemiras Shabbos Kehilchasa chapter 28 siff 30 with note 70 where we find the Chazzon Ish saying there is a problem of Boneh and R' Shlomo Zalman's who does not agree.
It should be noted that measuring for a mitzvah is allowed. So as the Shmiras Shabbos points out, measuring time for a sick person to eat is allowed. This is true even if he is not in danger.