How this timing is calculated
The local sunrise-to-sunset interval is divided into eight equal parts. Each weekday has one assigned part: Sunday the eighth, Monday the second, Tuesday the seventh, Wednesday the fifth, Thursday the sixth, Friday the fourth, and Saturday the third. The length is near ninety minutes when daylight lasts twelve hours, but it changes with season and latitude.
This page uses the daylight-eighth or Yamardha method described by Drik Panchang. The same publisher notes that other Rahu Kalam methods exist. This is a historical timing convention used in some Hindu calendars. It does not calculate a physical period of Rahu rising, an astronomical force, or a guaranteed result for an activity.
Why location matters
Sunrise and sunset change with date and coordinates. A generic weekday clock table assumes a twelve-hour day from 06:00 to 18:00 and can differ from your local result. The fixed offset is also essential: using an offset for a different place can put a valid solar crossing under the wrong civil date and clock label. At polar latitudes, a date may not have both sunrise and sunset; this tool leaves the window undefined rather than inventing one.
Sources: Drik Panchang Rahu Kalam method and Astronomy Engine 2.1.19 rise/set calculation.