Your one year field value is '2019-20' and you want this value to be in date format ? Could you explain a bit more. Usually in such financial year representation it is recommended to use dual(), may be something like:
Dual(YearField, Left(YearField,4)) as Year
This will give you value (text at at the UI) '2019-20' and a numeric value 2019 at the underlying value you can perform mathematical operation on.