29 March 2015
Once the period is closed you cannot pass the entries in 31/3/2014. you can use the fb50 with posting date 15/04/2014 ( actual bill accounting date) and pass the reversal entry. you can use the document date as 31/3/2014 and mentioned the provision reversal in the text field
or
you can try the t-code FB08 also to reverse the document with suitable posting and reversal reason