According to rfc3659 and http://www.nsftools.com/tips/RawFTP.htm#MDTM the month field for the MDTM command should be from 1 to 12, but ftpd uses the unconverted value from gmtime_r which returns 0 to 11 for the month.
Thanks! Fixed in git: commit 3bb858765e0524efdf2a05c8669ddbfde522d634 Author: Denys Vlasenko <vda.linux@googlemail.com> Date: Wed Jul 11 00:16:08 2012 +0200 ftpd: fix MDTM's month value