All calculations postulate point B[sub]g[/sub] to be positioned on central meridian of a timezone.[br][b][br]Summertime[/b] 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.[br][br]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 [b]was not corrected to 0hx' [/b]but [b]shown as 24hx'[/b] to point out this effect.[br][br][b]Timezones[/b] 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 [math]\pm[/math]20°, giving the difference of longitude to central meridian of a timezone.[br][br]Examples:[br][br]a) Places in MEZ - zone: central meridian : 15° East[br]Exactly positioned on this meridian is only the town of Görlitz/Saxonia. correction: 0°0'.[br][br]Linz / Capital of GeoGebra: l[sub]g[/sub]=14°17' East = 15° [b]- 43'[/b] , i.e. correction: - 0°43' (west!)[br]Cologne:                                   l[sub]g[/sub]=  6°57' East = 15°[b] - 8°3'[/b],i.e. correction: -8°3' (west!)[br]Vienna:                                      l[sub]g[/sub]=16°22' East = 15°[b]+1°22'[/b], i.e. correction: +1°22' (East!)[br][br]b) For other timezones:[br][br]Moscow:               l[sub]g[/sub]=37°37' East = 30°+7°37'=2[math]\cdot[/math]15°[b]+7°37'[/b], i.e. correction: +7°37' (East!)[br]New York City:     l[sub]g[/sub]=74°0' West = -75°+1°0' = -5[math]\cdot[/math]15°[b]+ 1°0'[/b] i.e. correction:+1°0' (East!)[br]Mexico City:         l[sub]g[/sub]=99°9' West = -90°-9°9'= -6[math]\cdot[/math]15°[b]-9°9'[/b],   i.e. correction: -9°9' (West!)