The problem is that a US date can parsed by Excel as a UK date when the day is less than 13. When this happens Excel converts it to the localized UK serial (date) number.
So 03/19/2014 is obviously a US date of the 19th of March. However 05/03/2014 is ambiguous so Excel parses it the local date format as the 5th of March, rather than the US 3rd of May. Any formula has to check if Excel has stored the US Date as a UK date. A UK date will be stored in Excel as a number.
=IF(ISNUMBER(A2),DATE(TEXT(A2,"yyyy"),TEXT(A2,"dd"),TEXT(A2,"mm")),DATE(RIGHT(A2,4),LEFT(A2,FIND("/",A2)-1),MID(A2,FIND("/",A2)+1,2)))
(For a US date in cell A2 and PC date is dd/mm/yy).
If ISNUMBER
is true, the US date looks like a UK date and Excel has serialized it as a number. So can format the date as text and back to a date again. Note day is passed to the month parameter of the first DATE function to perform the conversion.
If ISNUMBER
is false, its stored as a string as Excel doesn't convert a date string with >12 months. So use string functions to split it up for the DATE function.