Timecorrection
All calculations postulate point Bg to be positioned on central meridian of a timezone.
Summertime is not used in all countries and starts/ends not on the same day every year. Therefore it's to be activated by a special checkbox if needed.
In high latitudes situated southwards near to the border of midnightsun it's possible that sunset takes place after midnight in the next morning. Therefore indication of time was not corrected to 0hx' but shown as 24hx' to point out this effect.
Timezones geographically have a width from 15° but often extend further because their borders are those of countries. The slide "west - east" therefore is dimensioned with 20°, giving the difference of longitude to central meridian of a timezone.
Examples:
a) Places in MEZ - zone: central meridian : 15° East
Exactly positioned on this meridian is only the town of Görlitz/Saxonia. correction: 0°0'.
Linz / Capital of GeoGebra: lg=14°17' East = 15° - 43' , i.e. correction: - 0°43' (west!)
Cologne: lg= 6°57' East = 15° - 8°3',i.e. correction: -8°3' (west!)
Vienna: lg=16°22' East = 15°+1°22', i.e. correction: +1°22' (East!)
b) For other timezones:
Moscow: lg=37°37' East = 30°+7°37'=215°+7°37', i.e. correction: +7°37' (East!)
New York City: lg=74°0' West = -75°+1°0' = -515°+ 1°0' i.e. correction:+1°0' (East!)
Mexico City: lg=99°9' West = -90°-9°9'= -615°-9°9', i.e. correction: -9°9' (West!)