The explanation of calculating שעות זמניות based on שקיעה האמיתית (and do I presume that there's a similar differece in regards to הנץ החמה?) explains a question I had since the switch to Standard Time this year:
Apparently some Android App has a different סוף זמן קריאת שמע for the Alter Rebbe and the GRA. This was something I hadn't heard of or seen before, and was trying to understand/find a source.