Assuming your fiscal year is different from the calendar year. For example if the valid dates are 01-Jun-17 to 31-May-18. If this is fiscal year 17, then Year(01-Jun-17). If this is fiscal year 18, then Year (31-May-18)
You need to know 2 things. When the fiscal year starts and if the fiscal year is for the current year for the next year. In your example 15-10-2015 is Fiscal year - 2016, so your fiscal year is for the next year, 2016 instead of 2015.
For any date, if it is less than Oct 15th, the fiscal year is that date, Year(Date). If the date is greater than or equal to Oct 15th, the fiscal year is the year of the date +1, Year(Date) +1.