Which time zone does the validity follow?

Check the plan detail page at purchase and the order confirmation email. Both usually state the time unit clearly. If it is still unclear, contact support before you travel.

Generally not, since the clock usually starts on the first successful network connection, not on installation. Always confirm against your specific plan's terms to be sure.

Open the app and check the recorded activation time against your actual landing time. On a day-based plan, if the two straddle midnight, that day may well have counted as day one even though little of it remained.

No, most run as one continuous block for the whole trip and do not pause at a border crossing. Add up the whole trip's days before choosing the plan length.

No, leftover data disappears at expiry and is not transferred to a new plan. A new plan starts its own data allowance and validity clock from scratch.

Check the remaining-time counter in the app and top up or buy a new plan before the current one expires. Do not leave it to the last moment, as connectivity can gap while you are in the process of buying.

Neither is universally better; it depends on your trip pattern. Hour-based counting is fair to whatever time you activate, while day-based counting is often easier to reason about when a trip has a clear start and end date.