Here is an idea to play with:

=IF(DATEDIF(I8;G8;"m")=0;"";DATEDIF(I8;G8;"m"))+IF(DATEDIF(I8;G8;"md")<15;0;1)&" months"

It will add 0 months to the initial month calculation if the days number are <15, and add 1 month if days are>=15
instead of the above formula, you can use:
=IF(DATEDIF(I8;G8;"m")=0;"";DATEDIF(I8;G8;"m"))+CE ILING(DATEDIF(I8;G8;"md")/30;0,5)&" months"
to get decimal results...and so on

Change ; with , to match your local settings