The code you have just compares the numeric day of the month; a generally accepted response is to convert the date to filetime and run the comparison that way. Without a day in the Date constructor, it will default to the first day of the month. Also, the month in the constructor is zero-indexed, so Jan will be 0, Dec will be 11.
You can also de-clutter the code quite a bit using this mechanism.
var user = cmbUser.GetValue();
var wholesaler = cmbWholeSaler.GetValue();
var today = new Date().getTime();
var uploadDate = new Date(cmbYear.GetValue(),cmbMonth.GetValue() - 1).getTime();
If cmbMonth is also zero-indexed, remove the "-1".
You also need to modify your checks, since month CAN be 0. If you are doing null checking (and it looks like you are), you can simplify this check with:
else if (!month) { alertText('" + MessageChanger.GetErrorMessage("AP092") + "');}