Pivot table grouping issue

[goodmorningboy]

Hello all, new user here, great community!

I'm having a problem with a pivot table I created.
Very quickly, I have this rows (those are days, number format)

-3
-2
-1
0
1
2
3

I'd like to group them by 7 (a week), no problem with that;
BUT I need to keep the 0 separated, something like this...

< 0
0
1-7
8-14
...

Is it even possible?
ty in advance