Why do some string values give a correct date time and some not after a Excel data import?

Hi guys

I am noticing strange behavior when importing date times from xls data import.

First of all:

1 - unwanted behaviour that strings with datetimes are converted from MM/dd/yyyy to MM/dd/yy? Why is this happening and how can I set this to yyyy?

2 - The date times in xls excel file are strings, as dd/MM/yyyy. When the raw data is imported with the use of Excel importer, I got a totall new datetime value with 15 year difference. While other string datetimes are taken over correctly to mm-dd-yy.

anyone an idea for this strange behavior?