[Koha] Sorting by acquisition date doesn't work in advanced search

Hello, Sorting results by acquisition date doesn't work in advanced search (selecting "Newest to oldest" or "Oldest to newest" give results in the same order) on my installation (Koha 3.01.00.100, Zebra 2.0.42, marcflavour=UNIMARC). I suppose that the problem is Zebra-related. It seems that acquisition date is stored in the 8 first characters of field 100$a (for example: 20091224). In "etc/zebradb/ccl.properties", I read : Date-of-acquisition 1=Date-of-acquisition acqdate Date-of-acquisition But nothing about Date-of-acquisition in "etc/zebrardb/marc_defs/unimarc/biblios/record.abs" for the field 100$a. Can anyone give me a hint ? Thanks in advance, Alex