An observing night crosses a calendar boundary. A session starting at 23:30 and ending two hours later ends on the next date, even though observers may call it the same night. Keeping the date and zone together prevents avoidable mistakes.
Use the location's clock
NightSky uses the selected location's IANA time zone. A city in another country does not use your device's clock. The chosen-time list shows the local date, clock time, abbreviation and numeric UTC offset. The plan also prints UTC windows for comparing notes with someone elsewhere.
During the autumn daylight-saving change, a clock hour can occur twice. Two options with the same hour but different offsets refer to different moments. During the spring change, some local times do not occur at all. The list follows actual elapsed time, so it does not create those missing hours.
Check the start and the length
Select A chosen start time and then a session length. The sky diagram uses a sampled moment near the start; the observing assessment considers the eligible part of the whole interval. A two-hour plan can include both usable darkness and bright twilight. The result tells you how many minutes meet your target's darkness rule.
For custom coordinates, choose the time zone used at the observing spot. Coordinates alone do not establish the legal clock. If you are close to a border, confirm the zone with a local authority or your accommodation before relying on a departure time.
Sources and review
Reviewed October 22, 2026 by PiSpace Sky editorial. Date-sensitive details can change.