This is a puzzle I have to work out every year when I publish My Calendar
If one follows the astronomical date and time
precisely, there will sometimes be 16 days between Uposatha days, but there should be either 14 or 15 days. If the full moon in the UK is at 07:12 am on the 15th January should we observe the Uposatha day on the 14th or the 15th?
I suspect that the Forest Sangha calendars follow the Thai calendar
dates, rather than adjusting for the location.
As noted above, it is not that important for lay people. Most of the temples I know of celebrate the Uposatha days on the nearest Sunday as that is more convenient for lay people.