Michael R. Stern 
Sand Storm [EPUB ebook] 

After discovering a portal through space and time, Fritz helped the United States resolve a foreign crisis. Now, the president wants him to avert a nuclear threat in the Middle East.
For Fritz, travel to the past is entertaining and enlightening. Saving the world is something else altogether.
Racing against time, Fritz tries discover the portal’s secrets while helping the president. But does using the portal have long-reaching consequences he doesn’t yet understand?