In the UK it’s pretty clearly spelled out (although not always perfectly applied, I’m sure there’s still the odd boss trying it on).
Your working day starts when you arrive at your contracted place of work, and are ready to start work. Not when you walk in the door, before having a cuppa or breakfast in the office kitchen. Not after your computer has booted up and is ready for you.
If you have multiple places of work, or are travelling away from your contracted place of work, then your working day starts the moment you walk out your door and leave home.
The end of the day is the same, if you’re in the office it ends and then you leave, if you’re working away it ends when you get home (so factor in travel time and leave site before then).
Whether or not you actually get paid for every hour is another matter, however. Salary vs hourly work. If you’re salaried it’s supposed to be give and take - however it’s ultimately up to you to take what you can to balance it out. Work isn’t going to offer you an early finish, not as easily as they’ll ask you to stay late.