MYOB Premier v12 when you are using DD-MM-YYYY as your date format, export to Excel day between 1-12 will display wrong information.
The simple solve solution is using MM-DD-YYYY then Excel will not got the problem, but your MYOB date display and invoice print out you may not fit for your business.
This problem is caused by MYOB Excel Template, I think this is Bug, just try to replace the template file using the Premier v13 version, that the problem is solved.
Template Download Link
MYOB Premier v13 Excel Template
MYOB Premier v12 Excel Template
After download and unzip the file, all template file included in zip file, let say report the name is accting1, then just replace new version accting.xlt is ok.
