The grains are just used for measurement. They drop down because of the gravity. Now that mechanism will only work for the earth as it is measured that the gravity takes an hour to pull down the sand grains in the hourglass, in the moon it will take more time for the gravity to pull down the sand in the hourglass this is because the gravity is less there,but time passes in the same speed in both places it isn't slow in the moon nor fast on earth . the time doesn't have anything to do with the hourglass, the hourglass is just an apparatus used for the measurement. The time just keeps on going but the hourglass is just made by seeing that it will always take an hour for the sand to drop in the hourglass if it is placed on earth.