A panchang is not a calendar. A calendar tells you the date; a panchang tells you what the sky is doing, and the two answer different questions.
The word is literal: five limbs. Each one is a measurement, and each one is taken at your own sunrise rather than at somebody else's.
The five
- Tithi — the distance between the Moon and the Sun, divided into thirty parts. It is the lunar day, and it does not begin at midnight.
- Vara — the solar day, sunrise to sunrise. The only limb most calendars get right, because it is the only one they have to compute.
- Nakshatra — which of the twenty-seven divisions of the sky the Moon is standing in.
- Yoga — the Sun and Moon added rather than subtracted, divided into twenty-seven.
- Karana — half a tithi. Eleven of them, four fixed and seven that repeat.
Why the ground matters
Every one of those five is read at sunrise, and sunrise is a local event. Ujjain and Guwahati do not share one. A tithi that ends at 07:12 has already turned for a reader in the east and has not yet turned for a reader in the west, and a printed almanac has to pick one of them and be wrong for the other.
This is the ordinary reason two panchangs disagree, and it is not a disagreement about the tradition. It is a disagreement about where you are.
A tithi is a measurement. If two people report different measurements, ask what each of them measured.
What VEDAUM does with it
The five limbs here are computed from the Swiss Ephemeris for the place the reader is actually in, with the Lahiri ayanamsa, at that place's own sunrise. Nothing is looked up from a table, and the end time of each limb is found by bisection rather than rounded to the nearest hour.
You can check it. Take today's tithi from any almanac you already trust, set the same place, and compare. That is the only argument a computed panchang can honestly make, and it is the one worth making.