Microsoft Excel…it loves to format dates in a crazy way. By default it will do day dash 3 digit month name and 2 digit year like this: 13-Sep-2016. Not helpful. In the U.S. your first instinct would be to change it to 2-digit day dash 2-digit month dash 2 or 4-digit year: 10/13/2016. Salesforce.com does not like either date. Googling ‘Date: invalid date’ might lead you down the path of adding time to your date…don’t do it! Even for Date/Time fields if you don’t have a specific time already do the following…
Solution: Take your dates and do a custom format using the following: yyyy-mm-dd