In order to properly observe the laws of niddah, it is important to know the precise times of sunrise (henetz hachamah, often called netz) and sunset (shekiat hachama or shekiah). They apply in the following situations:
- The hefsek taharah examination and the bedikot (internal examinations) of the shivah neki'im (seven blood-free days) must be performed during the day, before sunset.
- The onot perishah (periods of separation) observed on vesatot (days when menstruation is anticipated) last either from sunrise to sunset or from sunset to sunrise.
We have linked to two sites (http://www.kashrut.com/zemanim and http://www.myzmanim.com) that provide the halachic sunrise and sunset times for any location on any date. These may differ slightly from those posted in your local newspaper. One should, however, be cautious even when relying on the halachic times, as halachic authorities differ slightly as to how they are calculated. Altitude may also have a slight effect. In practice, one should leave a margin of a few minutes around the posted times. If a halachically significant event occurs very close to the posted time – for example, if a woman did a hefsek taharah or discovered that she was niddah precisely at sunset – a rabbi should be consulted.
If it's not too cloudy, one can also check when the sun sets by direct observation. Halachic sunset occurs when one can no longer see any part of the sun.