The short version
- BS and AD are different calendar systems; conversion is not a fixed year offset.
- A converted weekday should agree in both directions.
- Tithi is astronomical and locality/tradition-sensitive, unlike civil date conversion.
- Important legal or filing dates should be checked against the issuing authority’s notice.
Why a fixed offset does not work
Bikram Sambat month lengths vary by year. Subtracting a fixed number of years or days can produce the wrong date, especially near month boundaries. A converter needs a year-specific BS calendar table or a validated conversion library.
The result should be reversible: converting a supported BS date to AD and converting that AD date back should return the original BS date. Weekday display offers another useful consistency check.
Input formats and validation
A friendly converter can accept hyphen, slash or dot separators while still requiring an unambiguous year-month-day order. It should reject impossible months, invalid day numbers and dates outside its supported range instead of guessing.
For copied documents, inspect whether the date is written in BS or AD before entering it. A value such as 2083-04-05 is clearly BS in ordinary Nepal use, while 2026-07-20 is AD, but the calendar choice should remain explicit.
Weekday and date difference
The weekday belongs to the underlying civil day and must agree whichever direction is used. A date-difference tool should compare normalized dates and explain whether the count includes or excludes the start date.
Relative labels such as “in three days” are convenient, but formal documents should retain the complete converted date.
Tithi and occasions are supplementary
Tithi is based on the angular relationship of the Sun and Moon, not only the civil date. Results can vary with calculation method, location, sunrise convention and tradition. An occasion can also span or shift relative to a civil date.
For this reason, Nepal Toolkit treats Kathmandu Panchanga information as supplementary. It shows a key occasion only when identified and does not turn absence of occasion data into a failed date conversion.
When to verify elsewhere
For court deadlines, tax notices, visa documents, birth records or another consequential use, compare the result with the issuing authority, an authoritative calendar or the original record. A converter is useful evidence of calculation, not authority to amend the source document.
Where to check the details
For an important payroll, filing or financial decision, open the source itself and check whether anything has changed since this guide was updated.
- Nepal Toolkit conversion methodology
Testing, source hierarchy and correction process used by the toolkit.