to_timezone
Description:
Create a new DateTime corresponding to the same instant in time as 
      this, but in the time zone tz.
This call can fail in the case that the time goes out of bounds. For example, converting 0001-01-01 00:00:00 UTC to a time zone west of Greenwich will fail (due to the year 0 being out of range).
Parameters:
| this | 
           a DateTime  | 
      
| tz | 
           the new TimeZone  | 
      
Returns:
| 
           the newly created DateTime which  should be freed with   |