How accurate is it?
Short answer: for published years Tithi Patro uses the official month lengths. For later years it calculates them with the same method, which reproduces 99.3% of past months exactly.
How the Nepali calendar is made
A Bikram Sambat month starts when the sun enters the next sign of the zodiac (the sankranti). That is why months run from 29 to 32 days and change from year to year. The Nepal Panchang Nirnayak Bikas Samiti publishes the calendar, and most apps ship only the published month lengths as a table. When the table ends, so does the calendar.
What we found
Modern astronomy (drik ganita) reproduces only about 81% of published month starts. Using the Surya Siddhanta sun, with the classical rule for Karka and Makara sankrantis at night, reproduces 99.3%. Tithis tell the same story: Surya Siddhanta matches 152 of 153 published days, modern positions only 126. So the official Nepali calendar follows the Surya Siddhanta, and Tithi Patro uses the same method.
The few months that differ all have a sankranti within 25 minutes of midnight, sunrise or sunset. That size of difference comes from the traditional calculation tables, which cannot be reproduced exactly. Days like these are flagged as lower confidence.
Sources and labels
- Official: BS 1975 to 2083, from published calendars.
- Standard table: BS 2084 to 2100, from three open tables cross-checked. Where they disagree (8 years), the majority row or the one that matches the calculation is used.
- Computed: every other year, from BS 1 to 9999. Several centuries from today sunrise can only be estimated, so a tithi that changes near sunrise may be off by a day there.
Results by rule
| Model | Day boundary | Matches |
|---|---|---|
| Surya Siddhanta | Classical night rule | 1496 / 1512 (98.9%) |
| Surya Siddhanta | Classical night rule, historical time | 1502 / 1512 (99.3%) |
| Surya Siddhanta | Midnight | 1436 / 1512 (95.0%) |
| Surya Siddhanta | Sunset | 1131 / 1512 (74.8%) |
| Modern (drik) | Classical night rule | 1110 / 1512 (73.4%) |
| Modern (drik) | Midnight | 1102 / 1512 (72.9%) |
| Modern (drik) | Sunset | 1228 / 1512 (81.2%) |
The months that differ
| Month | Published start | Calculated | Sankranti (Nepal time) |
|---|---|---|---|
| Mangsir 1989 | 1932-11-16 | 1932-11-15 | 1932-11-15 23:58 |
| Shrawan 2024 | 1967-07-16 | 1967-07-17 | 1967-07-17 05:24 |
| Jestha 2035 | 1978-05-14 | 1978-05-15 | 1978-05-15 00:09 |
| Magh 2035 | 1979-01-15 | 1979-01-14 | 1979-01-14 17:02 |
| Falgun 2046 | 1990-02-12 | 1990-02-13 | 1990-02-13 00:20 |
| Shrawan 2051 | 1994-07-16 | 1994-07-17 | 1994-07-17 05:19 |
| Jestha 2062 | 2005-05-14 | 2005-05-15 | 2005-05-15 00:05 |
| Chaitra 2066 | 2010-03-14 | 2010-03-15 | 2010-03-15 00:14 |
| Falgun 2073 | 2017-02-12 | 2017-02-13 | 2017-02-13 00:01 |
| Magh 2093 | 2037-01-15 | 2037-01-14 | 2037-01-14 17:29 |
Checked on 2026-10-28.