UPDATE!
After numerous attempts at the combos mentioned above (all of which are surely valid), I had been breeding an Air Dragon with a Moss Dragon (thinking it would yet again not be a Sandstorm). The incubation time was indeed only 2 hours and the egg is purple with silver on it!!
Try my combo or any of those listed by Emma. I think the Sandstorm may not be a guarantee on the first go, but you can get it.
--->My problem now is that I have an air boost and an earth boost, and when I place the Sandstorm dragon in either habitat (those are the two), I get a negative for both! But no other habitats on either island (the air and earth are on separate islands) are interfering! This is one of the trickiest parts of the game is figuring out what habitat goes where and what dragon is in it's best habitat so as not to get a negative!! Grr! Any advice would be of great help!
Just made level 19 too!