Can I use Outlook or Apple Calendar?

Yes. Your calendar link puts your booked signings in Outlook, Apple Calendar on a Mac, iPhone or iPad, or any calendar app that can subscribe to a calendar by its address: they show there, and you change them in Hand & Seal. Make it in Settings → Integrations, under Outlook, Apple Calendar and others — see Show your jobs in Outlook, Apple Calendar or another calendar.

Google Calendar is the one calendar that connects both ways by itself: your booked signings are written to it, and its busy times keep hours off your booking page — see Connect your Google Calendar.

Also good to know:

• See your signings in your phone's calendar app. In the phone app, turn on More → Settings → Calendar → Apple Calendar on an iPhone (This phone's calendar on Android): your booked signings go into a “Hand & Seal jobs” calendar in the phone's own calendar app — on an iPhone, Apple's Calendar — kept up to date, with no account to connect. On Android it stays on that one phone. On an iPhone it's kept in the same account as your default calendar, so with iCloud it shows on your other Apple devices too. See Put your jobs in your phone's calendar.
• Keep clients off the times you're not free. On the phone app, turn on More → Settings → Calendar → Busy times from this phone: when you're busy in the calendars on that phone you choose — Apple Calendar, Outlook, or any other you keep there — those hours stay off your page. Only when each event starts and ends is sent, each time you open the app; never what it is. See Put your jobs in your phone's calendar. Set the hours your page offers at all in Settings → Hours, under When you work.