Between European countries, there are only a few hours of time difference, which do not need to be particularly considered; insulin injections can be moved directly to the local time. For intercontinental trips, time zone differences are larger. A time difference of more than 3 hours should be taken into account in insulin dosing.
The necessary changes depend on the injection program and the insulin preparations used. For trips under a week, it is generally advisable to administer basal insulin at the home country's injection time. For example, basal insulin injected around 21–22 in Finland can be injected around 14–15 in New York (7-hour time difference). Mealtime insulin is not affected by the time difference—it is always injected before eating.
Planning the insulin injection schedule should be done well in advance of the trip together with the treating doctor or diabetes nurse. A written plan of the injection schedule can be made in advance, noting both the home country and destination country times, flight schedule and duration, and planning the insulin injection times with any changes.
In insulin pump therapy, it's usually sufficient to set the pump's clock to the destination country's time upon arrival and to revert it to the home country's time upon return.