When adding a membership, via the "Add Membership" task, you might encounter the error, "Conversion from string "1/13/2017 13 2017 12:00AM" to type 'Date' is not valid. when adding a membership". The date in the error will change depending on the day you are doing this.
This is a result of using an older out of date add data form for memberships in a user defined data list. Should you encounter this scenario please review any add data form specs and ensure that you're using the new/correct form of 'Membership Dues Add Data Form'.