So how do you overcome this? How to derive the last day in that period? Well the easy way is to use this built-in ABAP function in your transformation rules / or transfer rules etc:
CALL FUNCTION 'LAST_DAY_IN_PERIOD_GET'
EXPORTING
I_GJAHR = your_fiscal_year
I_MONMIT = 00
I_PERIV = your_fiscal_variant
I_POPER = your_fiscal_month
IMPORTING
E_DATE = output_last_day_of_period
EXCEPTIONS
INPUT_FALSE = 1
T009_NOTFOUND = 2
T009B_NOTFOUND = 3
OTHERS = 4
.
IF SY-SUBRC <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
* WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
ENDIF.
result = output_last_day_of_period.
So how to use this function? You need to replace the following fields which is highlighted in bold above, 'your_fiscal_year', and 'your_fiscal_month', should be replaced by the fiscal year and fiscal month respectively, in this case, you can derive this from fiscper from datasource. Remember, that 'your_fiscal_month', must be in 'financial format', meaning that its length is 3 and not 2 as usual. Remember to also populate the 'your_fiscal_variant', without this, the program will return with an exception error most likely.
This function will return 'output_last_day_of_period', and you can use this value to populate your calday in the transformation rules / transfer rules as you like!
Presto! That's easy isn't it?