I have a dataframe like this:

```
   Allotment    Date        NDII_Mean
   Arnstson     19900619    0.073023
   A_Annex      19900619    0.131290
   Arnstson     19900620    0.045553
   A_Annex      19900620    0.688850

```

and I want to group by `Allotment` and then subtract the `19900620` dates by the `19900619` dates. 

I want my output to look something like this:

```
Allotment      NDII_Mean
Arnstson       -0.02747
A_Annex         0.55756 

```