I get this error, I presume it has something to do i with the time periods. Can you help.

Your taiff that you are trying to validate links to 1 or more calendars.
The calendars need to be validated before the Tariff can be validated.
To see which calendars are used by the tariff, you can go to the overview screen.

Even more specific, a calendar needs to cover all the periods that the tariff covers.
It can cover more than the tariff, but not less.
So this error is when the tariff period covers areas that are not covered in the calendar.