How to compare panchang software
Panchang software differs in its ayanamsa, sunrise convention, Moon model and rules for end times. A fair comparison starts by matching these settings before comparing the results.
What it is
Check that both tools use the same ayanamsa and city coordinates, the same time zone, and the same sunrise altitude. Then compare the tithi, nakshatra and yoga end times for the same date and place.
How it is worked out or read
Small differences of a few minutes in end times are expected between good calculators, because their models differ. Larger differences usually come from a setting or a rule, not from the arithmetic.
Where it appears on this site
The site's method is on the method page. The verification article describes the comparison the site will publish.
The astronomy is rarely the problem
Modern astronomical libraries compute the positions of the Sun and Moon far more precisely than any panchang needs. The library used on this site is accurate to well under an arcminute for the Sun and Moon over the dates people use. Swiss Ephemeris and similar libraries used by other software are more precise still. So when two panchang programs disagree, the cause is almost never the underlying astronomy.
Where differences come from
The common causes are conventions and inputs. The ayanamsa shifts every sidereal position: the difference between Lahiri and Raman, for example, is well over a degree, enough to move a nakshatra end time by a couple of hours. The location matters for sunrise and every sunrise-based value. The definition of sunrise, whether the upper edge or the centre of the Sun's disc, and whether refraction is included, changes sunrise by a minute or two. And the time zone, including daylight saving abroad, must be right.
How errors grow in time
The tithi depends on the angle between the Moon and the Sun, which grows at about 12 degrees a day on average. So an error of one arcminute in that angle shifts the tithi end time by about two minutes. A nakshatra end time is similarly sensitive to the Moon's position. This is why even small differences in method can show up as visible differences of a few minutes in printed end times, while the underlying positions agree closely.
Rounding and display
Most panchangs display times to the nearest minute. Two programs that agree to within a few seconds can still show times a minute apart because of rounding. When comparing, treat differences of one or two minutes as agreement, and look for causes only when the differences are larger.
Checking a program
A good test of any panchang software is to compare it with a published drik almanac for the same city and ayanamsa on several dates, including dates near festivals and eclipses. Agreement within a few minutes shows that the software and the almanac are using compatible methods. This site's Method page states the conventions it uses, so that such checks can be made.