I think I was helping you with your workflow yesterday haha, but I also did calculations with dates for a sign-in log where I wanted a view of all logs by work week so it would say "Week of mm/dd/yyyy" and list all sign-ins.
Anyways, this may help you with your calculations. I did
2-WEEKDAY is something I had to play around with until it showed the Monday of the workweek. You can try doing something like that, or use the msdn guide on calculated columns for SP2010 at https://msdn.microsoft.com/en-us/library/office/bb862071(v=office.14).aspx
If I were to write your calculation, I'd write
=IF([Start Date & Time]-[Today]<=7,"Major","Minor")
Notice that your Major and Minor need to be strings.
If that doesn't work, you can try doing something like I did for the "Week of" calculation
EDIT: I just read the msdn article and looks like
=DATEDIF will do the trick to calculate the difference in days, but you will probably need to throw an
IF outside of the
DATEDIF to produce the "answer" (Major, Minor) to your condition