i am 90% sure Jelena. i say this because in the XML file the field is called MES and this is translated to MONTH not PERIOD and the allowed values for MES are 1 to 12. if you can send it by period you will use values from 1 to 16 or 1 to x depending on you fiscal year variant.
For example , at december 31th 2015 if you sent the information for MES 12 the program will take period 12 and won't include movements from periods 13 to 16 done in december 31th as posting date. the transaction has as allowed values 1 to 12 the same as months but is getting periods from fiscal year variant and not natural months and this seems that something is not ok