What is Grahan (an eclipse)?
A Grahan is an eclipse — Surya Grahan (solar) when the Moon comes between the Earth and the Sun, and Chandra Grahan (lunar) when the Earth's shadow falls on the Moon. In Vedic tradition an eclipse is a spiritually charged but inauspicious time; auspicious work and temple worship are paused, while chanting, meditation and charity are encouraged.
Sutak Kaal
Sutak is the impure period observed around an eclipse. It begins 12 hours before a solar eclipse and 9 hours before a lunar eclipse, and ends when the eclipse ends (moksha). Crucially, Sutak is observed only where the eclipse is actually visible — if an eclipse is not visible in your city, no Sutak applies there. During Sutak, eating, temple worship and new ventures are avoided; the elderly, the ill, children and pregnant women are exempted from fasting.
How the timings are set
Whether an eclipse is visible, and its exact timings, depend on your location, so set your city above. The Sutak window is then shown in your local time; where the eclipse is not visible, the tool notes that Sutak does not apply.