Excel 2007 :: Conversion Of Text Into Date Format

I have a question about conversion of a text format into date format.

2 computers, the first windows vista, the second Windows 8, both Excel 2007.

The purpose is to calculate the difference between 2 dates with hour:minutes:seconds, like for example:
18/12/2012 19:20:00 and 18/12/2012 19:23:03.

So in this example the result should be 00:03:03.

Both dates are formatted with 'personalized' : dd/mm/yyyy hh/mm.

In the first computer everything works like it should, but in the second it is apparently impossible to transform the first of those date h:m:s which is originally in a text format into a date format. I tried all the methods which i found to transform a text date format into a date , like multiplication with 1, Datevalue and others, but nothing seems to work, the result is always "#Value".