How the two windows are found
The daytime from local sunrise to local sunset is divided into eight equal parts. Yamaganda and Gulika Kalam each receive one part, selected by weekday. On Sunday, Yamaganda takes the fifth eighth and Gulika takes the seventh. On Saturday, Yamaganda takes the sixth and Gulika the first. Because daylight varies with date and place, a universal 90-minute clock chart is only a worked example.
Drik Panchang's Sunday almanac lists both windows with sunrise and sunset for Mulki. Its Saturday example shows Gulika at sunrise. The calculator follows this daylight-eighth convention; it does not calculate an orbit of Yama or a physical effect of Gulika. It returns a local calendar interval, not a recommendation or prediction.
What the local clock assumes
The bundled Astronomy Engine models the apparent Sun rising and setting at zero metres with standard atmospheric refraction. A mountain horizon, buildings, unusual refraction, or a regional almanac's solar convention may produce a different observed time. The entered UTC offset remains fixed for the entire date; check its historical daylight-saving status before comparing another calendar. At polar latitudes, a date can lack a sunrise or sunset. The tool then reports no window instead of assigning an invented clock time.
Sources: Drik Panchang Sunday almanac, Tuesday almanac, Saturday almanac, and Astronomy Engine rise/set API.