There are selectable values of Horizon parameter: 1) use Celestial (Astronomical, True) horizon 2) use Earth-sky (Visible, Local) horizon
6:01 is sunrise in Earth-Sky (Visible or Local) horizon 7:00 is sunrise in Celestial (Astronomical, True) horizon. Note: daytime longs 12 hours and night longs 12 hours, i.e. "Daytime equals Night time" 13:00 is Noon time - it is the same for both cases
Earth-Sky Horizons
Show/Hide to read more about Earth-Sky Horizons...
The local horizon, also called the geometrical horizon, is the visible boundary between the Earth and sky. The local horizon may include trees, buildings, and mountains. The geographic horizon is the apparent boundary between the Earth and sky. Mountains, trees, and other elevated features are not considered part of the geographic horizon. The sea-level horizon is the geographic horizon at sea level. One of the best places to see the sea-level horizon is a beach. The ocean and the sky provide a clean, flat line where the Earth seems to meet the sky. If you're standing on the beach looking out at the sea, the part of the sea that "touches" the horizon is called the offing. read more
Celestial Horizons
Show/Hide to read more about Celestial Horizons...
Celestial horizons are used by astronomers, for the measurements of the position of the Earth relative to the rest of the sky. The celestial horizon is a flat passing from the earth’s centre perpendicular to the zenith-nadir axis. The visual horizon approaches this plane at the earth’s surface. The astronomy horizon is the type of celestial horizon whose imaginary horizontal plane is always at a 90-degree angle from the observer's zenith. Celestial horizons are perfect circles that surround the observer. The true horizon is the theoretical plane that passes through the centre of the Earth, its radius is perpendicular. From orbit, the true horizon is spherical, following the shape of the Earth. The altitude of a celestial body is calculated using the celestial horizon as a reference point. Celestial horizons are called rational horizon to compare sensible horizon. read more
Vaishnava scriptures say that Daytime equals Night time, i.e. daytime longs ~12 hours, night time longs ~12 hours. So, the whole day longs 12 + 12 = ~24 hours.
First, there are always twelve hours of daytime and twelve hours of night-time at the Equator (read more about the Equator). Second, the dates on which day and night are each 12 hours occur a few days before and after the Equinoxes (read more about length of Day and Night at the Equinoxes). Finally, Vaishnavas use Celestial horizon for calculation, having both Daytime and Night time equal always.
Well, let's take a look at CC Madhya 20.390 Translation of His Divine Grace Shri Shrimad A.C. Bhaktivedanta Swami Prabhupada: The day and the night are divided into eight praharas — four belonging to the day and four belonging to the night. After eight praharas, the sun rises again. Daytime (4 praharas) = Night time (4 praharas) Daytime (12 hours) + Night time (12 hours) = day (24 hours = 8 praharas) prahara ~= 3 hours In example above: Daytime starts at 7:00 (Sunrise) and ends at 19:00 = ~12 hours. Night starts at 19:00 and ends at the next Sunrise = ~12 hours.
Below will be a list of links and quotes from the holy scriptures, books and conversations which say that "Daytime is equal to night time", or "length of day = length of night = 12 hours", or in other words, they say about using Celestial timings (not Earth-sky timings). CC Madhya 20.390 (+ CC Madhya 20.389 + CC Madhya 20.388 ŚB 3.11.10 ŚB 6.8.20 Purport BG 8.17 Purport TBD...
This Vaishnava Calendar for ISKCON uses "Celestial sunrise" (Celestial horizon option) to calculate dates of events/holidays (as well as breaking fast timings). But in most calendars sunrise is calculated with Earth-Sky (Visible or Local) horizon. It is the case when your sun limb appears on the horizon.