Selected Answer
VBA manages dates as numbers, an integer for a day and a fraction for the time of day. To gain control of imported date strings you must convert the strings to numbers. In order to do so you must know the format. Hence, the best solution is not to import the strings. Can you get at the numerical values from which the strings were created?
Presuming that this isn't the case, presuming that all dates are of the same format, and that you have a lot of them, you can write a macro which determines the format. This will not be possible if the year is represented by 2 digits And you don't know its position (first or last). You can look for a date separator (dash, slash, period) if the separator isn't known before you start. With the separator you can split the date string into its 3 groups. The one with 4 digits is the year. You can look for a number greater than 12 in the middle group. If you find one the middle group has the day and the remaining is the month.
With this information you can create the DateSerial number which you can write into your imported data and, thereafter, issue your own date string in any format you like.
All of this might be done on the raw data before import or on the imported data immediate afterwards.