Date origin wrong if non-date in column

[GoogleCodeExporter]

As posted as a StackOverflow question, with sample file and code:

http://stackoverflow.com/questions/25158969/read-xlsx-reading-dates-wrong-if-non
-date-in-column

Basically, if there's a non-date in a column then read.xlsx will convert dates 
from using the excel 1899 epoch, rather than 1970 and so be 70 years in the 
future. Skip reading the non-date item and it all converts correctly.

Original issue reported on code.google.com by b.rowlin...@gmail.com on 6 Aug 2014 at 3:49