The aforesaid error "Invalid date"generally occurs if you are using the Tally Demo (Educational) version and the Vouchers in the Excel Sheet have DATE's other than 1st or 2nd (of any month).The Educational version of Tally accepts vouchers for the 1st and 2nd of each month. i.e. You cannot import vouchers having date other than the 1st or 2nd of any month.
Another reason could be that the date column in XLS contains DATE and TIME. ex: 2016-07-31 10:10:00. You must change it such that the XLS data contains a valid DATE.
The valid Date formats are:
dd-mm-yyyy
dd/mm/yyyy
yyyymmdd
yyyy-mm-dd
yyyy/mm/dd