I'm currently implementing the conversion between equatorial coordinates (object positions in catalogues) and horizontal coordinates (positions in the observer's coordinate system) and back. I need the universal time (UTC might be sufficient as we don't need to be overly precise here) and the observer's position for that. Both can be obtained with GPS, but there's one problem with that: GPS doesn't work inside buildings and I'm writing and testing the code inside. I will add a menu that allows to set both to sensible defaults or to manually entered values.

My goal is to have an automatic startup procedure that tries to obtain a GPS fix and switches to the default/manual mode if that cannot be obtained within a reasonable time. That should be ok for the location as I'm usually observing from the same place, but the time must be set manually nonetheless if there's no GPS available.

There are some very helpful sites that on those conversions:

Wikipedia article on Julian Day

Approximate Sidereal Time - US Naval Observatory

Local Sideral Time clock - an online calculator that is very helpful during debugging

Wikipedia article on Astronomical Coordinate Systems

It's kinda cool when your coding problems boil down to the fact that earth rotates.