This model is a remix from MakerPaul:
https://www.prusaprinters.org/prints/87021-tipping-egg-sand-timer-30-seconds

It is a printed sand-timer that fall when the time has passed and can be reset even before the whole time has passed.

In comparison with the initial file this model is fully 3D printed and it includes a M3 screw to fine tune the size of the opening. It is also smaller than the initial model.

It should be printed straight and without support. If you have difficulties with the overhang at the base of the model you can use support there but be cautious not to put any inside the timer itself.

The side of the control screw has graduations to help tune the size of the opening. When the tip of the screw is at the last graduation it is half closing the hole.

The sand you use is off critical importance. It should be thin enough to flow easily but the smaller it is the more sensitive it will be to humidity. I use a powder sieved between 150 and 100 mesh (ie around 100 to 150 micron) and had good results.

Video of the timer at:
https://www.youtube.com/watch?v=dkwy7PZ1ptg