Start with the time you can actually observe
Enter the first local evening and the hours available to you. A session from 20:00 until 04:00 belongs to the evening on which it starts. This makes the plan useful when you cannot stay outside for an entire night.
Choose the location’s IANA time zone rather than guessing a fixed offset. A city’s offset can change with daylight saving, and two destinations can have different rules. The planner calculates UTC instants first and presents the matching local times.
Choose your darkness and Moon rules
Astronomical darkness uses a Sun altitude at or below minus eighteen degrees. Civil or nautical twilight can be useful for brighter targets but is not equivalent to a fully dark sky. At high latitudes, some summer nights may never reach the selected limit.
The default Moon rule requires it below the geometric horizon. An illumination allowance can admit a faint Moon above that horizon. A raised horizon mask can approximate a blocked view, but moonlight scattered through the atmosphere can still affect the sky.
Treat the result as a planning window
The calculation samples five-minute slots and evaluates each midpoint. A start or end is approximate by one slot, not a precision rise or set prediction. Inspect your actual surroundings and allow time to set up before the planned interval.
Download the calendar events and the evidence CSV together. The calendar is convenient for scheduling, while the CSV explains which Sun, Moon and cloud values were used. Neither predicts light pollution, atmospheric seeing or whether a particular target will be visible.