scales problem

Hi I want (in the picture you can see better my issue) a gantt with the following scale format

2013

Jan | Feb | March | May | Jun | Jul | August | Sept | October | Nov | Dec

and with the possibility to resize a tasks by day with the drag and drop option (not with the modal), not by month or by week, by day, the user can move a task to starts on Jun 01 2013 or Jun 02 2013 or Jun 03 2013, I can do that with the following code, my issue is in the gantt, If I say gantt.getTask(3).start_date is Jun 01 2013 but in the graphics look like May 27 2013

This is my code

gantt.config.readonly = false;
gantt.config.columns =  [{name:"text",label:"Task name",  tree:true, width:'*'}];
[b]gantt.config.scale_unit = "day";[/b]
[b]gantt.config.step = 31;[/b]
[b]gantt.config.date_scale = " %M";[/b]
gantt.config.autofit = true;
gantt.config.subscales = [{unit:"year", step:1, date:"%Y"}];   
gantt.init("gantt_here", new Date(2013,0,1), new Date(2014,0,1));

If I change gantt.config.step to 30 this generate two months for months with 31 days.
Please help, Thanks