How to describe sand flowing through an hourglass
There's always the famous (and parodied) introduction to the soap opera The Days of Our Lives:
"Like sand through the hour glass, so are the days of our lives."
The usages that the Ngram viewer finds:
From The Hourglass of My Life by Caite Grace:
Like sand running through an hourglass, I can see my life slipping away in front of me
From Treating Addicted Survivors of Trauma by K Evans and J. M Sullivan:
The dragon is lying on sand, with more sand flowing through an hourglass to symbolize the passing of time.
From The House by the Sea: A Journal by May Sarton:
WHERE HAS the time gone like sand pouring too fast through an hourglass?
From Firehorse by D L Wilson:
This wasn't important work; this was nothing more than sand trickling through an hourglass,....
Sand also slips through an hourglass.
One of the meanings (mentioned below) of slip is to flow smoothly.
Also, one of its other definitions is elapse which applies to passage of time and nicely fits in the context.
M-W:
slip (slipped, slipping)
intransitive verb
1a : to move with a smooth sliding motion.
c : elapse, pass4a : to slide out of place or away from a support or one's grasp.
b : to slide on or down a slippery surface : slip on the stairs
c : to flow smoothly7: sideslip