Date/Time values are stored as Real Numbers internally. Formatting is done to display them in required format. Formatting will not change the data.
Change of date format takes place during data entry, depending on the 'Region & Language' setting of the Computer.
Assume that the data-entry field format is dd/mm/yyyy and you enter the date as 01/12/2013 for 1st December 2013 (date number: 41609). If the Region & Language setting is US format the date will be recorded as 12/01/2013 (12th January 2013, date number: 41286). This change will happen only on dates from 1 to 12 of any month.
Hope you have noticed the date number difference. After recording them wrongly doesn't matter which way you format it, it will be wrong, till you update them correctly.
I think it is important to change the 'Region & Language' setting of the computer first to a suitable Region and edit the dates in Excel itself, where ever necessary to make all of them to a common format.
You can change the date column into numeric format in Excel and upload into Access and change the Table column into Date/time format. The date will always remain as a Real Number internally in Excel/Access (Date/time - 41609.5 is 1st Dec 2013 12:00 Noon)