I faced a similar problem with the financial year running over two years.
I would suggest using your Start Date field as a Date field, so no need to convert it to an Integer. The calculated fields below work on Date fields and return the data as Text.
Here's the plan:
I used some calculated fields to grab the month and year - using the 'Created Date' as the date source field. My data is returned as text rather the integers, but maybe you can adapt your fields or the formula so it works for you?
To grab the year data (Year field)
To grab the month data (Month field)
This calculation looks at the Month field. If the Created date is in Nov 2017 or Dec 2017, the calculation will add +1 to the Period and return 2018.
If you want your Period/Financial Year to run from e.g. April then substitute the 11 for a 4 - in this case your financial year would run from 01/04/2017 - 30/03/2018. If a Created date was between 01/04/2017-30/03/2018 the Period returned would be 2018.
So in my case (using the Nov to Oct dates) the calculation will see that for a Nov 2017 date a +1 should be added to the year if the month is Nov or Dec.
List of Field Names
Created = Created Date field (for your problem, just read 'Created Date' as 'Start Date'
Month = takes the month from the created date and returns it as text
Year = takes the year from the created date and returns it as text
Period = calculation which uses the data from the calculated Month and Year fields and returns data as text