As per the hindu mythology one of the 8 segments of day is considered highly inauspicous and is termed as Rahu Kaal because of its association with Rahu the malefic god. The time is calculated by taking total time between sunrise and sunset and dividing it by 8.
As per the guidance of Astrologers, Rahu Kaalam is highly inauspicous and should be avoided for doing any critical tasks such as financial transactions or inaugration of business.
This app provides you when alerts when RahuKaalam has started. It is very useful if you want to avoid any inauspicious period.