How do I convert String to datetime or how do I replace their values with one correct value from ...

Even though they have the same InvoiceNo, they have different values for InvoiceDate. So how do resolve this through data cleaning? Like I want to convert “40885.7458333333” to an actual date but it seems impossible so the other option I have is to take the actual/correct datetime “2011-12-08T17:54” and replace it into all the InvoiceDate with the same InvoiceNo. How may I do that?