Fail to get data from date column in excel

[tna274]

Hi David! First at all, thanks for your package, it's very useful.
But, when I read 1 excel file (have column type = date), it get incorrect results.
Example:

• Excel: 06/11/2016 ==> get data to php: float(42532)
• Excel: 06/10/2017 ==> get data to php: float(42896)
  Have problem here?

[moux2003]

Maybe this could be solved by accepting PR #274 ?