Try using this formula:

```
=IF(OR(AND([Mil Status]="AGR",[Composite Score]>=90),AND([Mil Status]="DSG",[Composite Score]>=75)),TEXT([Previous Test Date]+365, "MMM YYYY"),(IF(OR(AND([Mil Status]="AGR",[Composite Score]>=75),AND([Mil Status]="AGR",[Composite Score]<=89)),TEXT([Previous Test Date]+182, "MMM YYYY"))))
```
<hr>

Also, you can add months & years in date as well. Check below documentation for more information & syntax:

[Calculated Field Formulas](https://docs.microsoft.com/en-us/previous-versions/office/developer/sharepoint-2010/bb862071(v=office.14))