Suomenlinna tickets are valid for 12 hours. The tickets are only valid on the Suomenlinna ferry services and they do not include a right to transfer to other public transport modes.
You can purchase Suomenlinna tickets in advance from service points, many kiosks and shops, as well as from some HSL ticket machines. Advance purchase tickets are valid from the time you show the card to a card reader at the beginning of your first journey. You do not need to press any buttons, just show the card to the reader. The card reader emits an audible and the display shows the ticket information. You can check the expiry time of ticket at any time by holding your card on the reading area.
Alternatively, you can buy a ticket for the ferry with your HSL card value or a single or day ticket using the HSL app. AB, ABC and ABCD tickets are accepted on the ferry.