Why the panchang uses sunrise
Most panchangs use the sunrise of the day as the point at which the day's values are fixed. This site follows the same convention.
What it is
The panchang day starts at sunrise and runs to the next sunrise. A tithi or nakshatra that ends at 10 in the morning is shown as the tithi of that day, and the next one begins after 10.
How it is worked out or read
The sunrise is computed from the city's latitude, longitude and the standard sunrise altitude of about minus 0.83 degrees, which allows for refraction and the Sun's disc. The sunrise time is local to the chosen city.
Where it appears on this site
The sunrise for each city appears in the panchang. Different traditions use different sunrise rules in some regions, which is noted on the method page.
Why sunrise and not midnight
The choice of sunrise is practical as well as traditional. In a society where the day's work and worship began at dawn, the sunrise was the natural boundary, and the calendar followed the working day rather than the arbitrary midnight. The convention also avoids the question of which midnight to use, since the sunrise of a city is a clear local event. Modern civil time has a different boundary, so the reader must always know which one a given panchang uses.
Sunrise differences between regions
Sunrise is not the same everywhere on the same date, because it depends on latitude and longitude. A city far to the west of its time zone sees sunrise later on the clock. Some regional traditions use a different convention for the start of the day, or take the sunrise at a slightly different altitude. These differences are the main reason that two panchangs for the same city can disagree by a few minutes. The site states the sunrise convention it uses, and the method page gives the altitude.
What to do when the day boundary matters
If an event falls near sunrise, check the time carefully against the panchang's sunrise value for that city. A tithi or nakshatra that changes just after sunrise may belong to the previous day in one tradition and the same day in another. In such cases record which convention you followed and why. The answer is less important than the consistency of the reading, since a reader who changes convention in the middle of a chart will reach a result that cannot be checked.
Checking sunrise for a city
A reader who wants to check the sunrise for a city can compare the panchang's time with a published sunrise table for the same date. Small differences of a minute or two are expected from different altitude conventions, and the reader should note which convention each source uses.
High latitudes
Far from the equator, sunrise changes quickly through the year and the day length varies greatly, so the panchang's sunrise-based timings shift more from week to week. Readers outside India should keep this in mind when comparing with Indian almanacs.