These are the sort of question that keep me up at night.

The person kept a sun dial and historical text on deck. How else? Maybe the person started time by setting the clock to just a random set of numbers. Maybe time invented the person and then time introduced timeself?

I wonder if I can make an igloo out of frozen giraffes...

He probably just checked his iPhone.

That's an easy one. They simply dialed 123 to connect with British Telecom's speaking clock. Way back then the toll for such a call was only a farthing, as opposed to the present day cost of nearly .40 pence.

In accordance with the sun. Of course in order to do this the person had to make their own scale for measurement. The person probably made up the units as well.

The sun. Just like using a sun dial, it would be straight up in the middle of the day (noon) and just go from there.

Astronomy.

Well he hooked up a bag of times and dates and selected one of them on the basis of probability

It all started with the railroad actually. At first all the towns would have their own time, which corresponded with where the sun was during the day. But then, when the railroad was built for trading and passenger transport, all of the town's had to have the right time in order to catch the train so everyone would link their clocks with the railroad time. There were actually "time wars" where towns would protest in favour of having their own time, because they had become personally attached to their autonomy of time. Trrue story.