I can't answer your question directly, but clearly there are various choices that make it a little more complex.
Full moon will fall at some particular time on a particular day in a particular time zone. But perhaps you want Uposotha to be the day when the moon is full before the end of the night
, rather than the day that the moon is full before middnight
... Lots of choices there.
And using the local time zone could be really confusing. So we might use different dates here in New Zealand from Thailand, for example? Should we do the calculation correcting for local noon, or using the time-zone time?
In practice, one would tend to simply use the calendar for the particular Nikaya one belongs to to avoid people showing up on different days!