If you are making a return journey, you can use your National Rail tickets and offer vouchers any day on or between your outward and return journeys. For example, if you arrived on Thursday and will return on Sunday, your tickets will be considered valid for the offers on Thursday, Friday, Saturday and Sunday. https://www.daysoutguide.co.uk/faqs
However, if you already left London, how to use it? Sometimes, if the ticket been stamped by the ticket inspector, then it will have the date of your actual travel.
https://www.daysoutguide.co.uk/faqs
Sometimes, if the ticket been stamped by the ticket inspector, then it will have the date of your actual travel.