This is a very "niche" question...Obviously, the accepted practice is to calculate halakhic times by שעות זמניות. However, a minority of Rishonim and Acharonim disagree with the whole concept and maintain that שעות שוות should be used. Over time, this opinion has been generally disregarded, except by Komarna Chassidim, who even today follow one of their Rebbes who was adamantly opposed to שעות זמניות. This is the background to my question. The (totally hypothetical) question itself: According to those who would propose the use of שעות שוות for halakhic times instead of שעות זמניות, when exactly would the day begin? When would you start counting the 60 minute hours to determine the deadline for Shema, Tefillah, eating hametz on Erev Pesah, etc.? I am assuming that beginning from an arbitrary point in time like 6AM makes no sense at all. And I know that with regard to Erev Pesah, the Terumat HaDeshen says to count two hours backwards from חצות, seemingly implying that we would have use the frame of reference of daylight hours for some things. But how would this be done in a way that is consistent?
Rav Yisroel Dovid Harfenes has a summary in his Yisrael vehaZemanim, vol. I pp. 55-59.
He brings down opinions that seem to hold that day always starts at 6am, and you count sixty-minute hours from there. However, as he points out, it's possible that those opinions stem from before clocks became standardized, when 12noon in every place was simply defined as the actual time of noon (when the sun is overhead).
Alternatively, there are those who say you count backwards from whenever noon is on a particular day. This seems to me like it could be done in a consistent manner.