Dates and Groovy

Hello, I'm having a hard time manipulating Date objects in Groovy implementation ... I basically want to get a timestamp of a month ago. For example I get a timestamp "200611" for now, and I want to retrieve the "200610" string. This is what I'm doing: <% timestamp = $xwiki.formatDate($xwiki.getCurrentDate(),"yyyyMM") oldtime = $xwiki.getDate().getTime()-(1000*60*60*24*30) def olddate = new java.util.Date(oldtime) oldtimestamp = $xwiki.formatDate(olddate, "yyyyMM") %> olddate contains a modified date (maybe the computation is wrong but for now it's not my problem). I get an error on last line, a NullPointerException occurring in the formatDate method. But I logged that olddate is not null (it contains a Date), so I don't really understand from where would come the NullPointerException ????? Many thanks in advance, Jérémie