SunSchedule
Daylight Duration and Daily Change Calculator
Estimate hours of daylight and daily daylight gain or loss in seconds for a chosen date, latitude and longitude.
Choose a date and enter coordinates to estimate daylight duration and its daily change. The starting values are a fixed, editable example, not your detected location or the current date.
- Choose the intended date.
- Enter latitude and longitude as decimal degrees. Latitude ranges from −90 to 90: north is positive and south is negative, so enter −35 for 35° south. Longitude ranges from −180 to 180: east is positive and west is negative, so enter −120 for 120° west.
- Read the two results: estimated daylight duration and daily daylight change in seconds. A positive change means increasing daylight; a negative change means decreasing daylight.
Daylight definition. The calculation uses the registered solar-events model with a solar altitude threshold of −0.833° for sunrise and sunset—an angle 0.833° below the horizon. Daylight duration measures the interval between those crossings and excludes twilight.
Date convention. The model samples the selected date at 12:00 Coordinated Universal Time (UTC). Results follow this UTC-noon convention rather than a user-selected local timezone. They describe durations, not local sunrise or sunset clock times.
Daily change: an illustrative example. The daily-change result comes directly from the model and is displayed in signed whole seconds. For illustration, −120 seconds means daylight decreases by 2 minutes, because 120 ÷ 60 = 2. A result of +120 seconds means daylight increases by 2 minutes. This measures change in daylight duration, not a shift in sunrise time.
Exceptional results. Where the model classifies the Sun as continuously above the sunrise threshold, daylight is 24 hours. Where it classifies the Sun as continuously below that threshold, daylight is zero hours. An unavailable result must not be interpreted as zero.
Accuracy. These are estimates under the model's horizon and atmospheric assumptions. Terrain, obstructions and actual atmospheric conditions can affect observed sunrise and sunset, particularly near polar transitions. Displaying whole seconds does not imply accuracy to one second.
For sunrise, sunset and twilight information, visit the SunSchedule homepage.