you shoud enter data according to your regional settings in control panel...
if you are in USA, 1/4/2012 means 4 jan 2012.
or, here in Romania, i use 4/1/2012 for 4 ian 2012.
Then, if you enter 4/1, this means you first enter month!!, that's why it shows 1 -apr. You can format the cell to show date in your custom way (the excel number for that date is the same, no matter how YOU CHOOSE TO SEE that number), but data should be entered according to reg. settings.