Methodology and Calculations

This page explains how Bengali Calendar Now builds the monthly grid and how we source observance dates. When there is any conflict, we follow our primary source.

Scope

• Shows Bengali date for each Gregorian day, with a simple tithi label for context.
• Publishes Ekadashi, Amavasya, Purnima, and festival listings as provided in Benimadhav Shil Ful Panjika.
• Provides a converter-style display for the Bengali month and day that users see in each cell.

Time zone and location

• All logic uses India Standard Time (IST, UTC+05:30).
• No daylight saving time is applied.
• The Bengali date model is configured for India. A Bangladesh start constant exists in code but is not active.

IST shift in code:

date.setTime(date.getTime() + (date.getTimezoneOffset() + 330) * 60 * 1000)

What is sourced and what is computed

Sourced from Benimadhav Shil Ful Panjika

• Ekadashi, Amavasya, Purnima dates and windows
• Festival and holiday lists
• Any special observance instructions

Computed in the app

• Bengali date for each Gregorian day, used for display
• A simple per-day tithi label for context
• Month headers and weekday rows

Formulas and constants

• Synodic month: 29.530588853 days
• Reference new moon JD: 2451549.5 (near 2000-01-06)
• Bengali year model length: 365.2587564814815 days
• India start JD: 1938094.4629
• Bangladesh start JD available: 1938094.483733333 (not active)

Gregorian to Julian Day

Function: ModernDate_to_Julianeday(y, m, d)

• Shifts Jan and Feb to months 13 and 14 of the previous year.
• Applies the Gregorian reform around JD 2299160.
• Returns a floating JD for use in calculations.

Tithi label per date (approximate)

This produces a tithi name inside each calendar cell. It is for context only. Official observance comes from the Panjika.

1. Compute daysSinceNewMoon = JD − 2451549.5. If negative, add whole synodic months until positive.
2. Set tithiLength = 29.530588853 / 30.
3. Compute tithiNumber = floor(daysSinceNewMoon / tithiLength) + 1, then wrap to 1..30 with ((tithiNumber − 1) % 30) + 1.
4. Map 1..15 to Shukla tithis and 16..30 to Krishna tithis.

Limits

• Uses a mean lunar cycle. Does not use true Sun and Moon longitudes.
• Does not apply sunrise rules or Viddha checks.
• Does not separate Smarta and Vaishnava rules.

Bengali date model

Used to print the Bengali day number and the two Bengali month names shown in the header.

Constants

• Start JD (India): 1938094.4629
• Year length in model: 365.2587564814815 days
• Cumulative month boundaries mas_len (days from year start):

index:  1      2       3        4        5        6
value:  30.93, 62.63,  94.00, 125.48, 156.49, 186.92

index:  7       8        9        10       11       12
value: 216.81, 246.32, 275.64, 305.09, 334.91, 365.26

Steps

1. Convert the Gregorian date to JD: nJD.
2. jddiff = nJD − startjd.
3. lastYearCount = floor(jddiff / 365.2587564814815).
4. mesh = startjd + lastYearCount * 365.2587564814815.
5. Find month i such that nJD is in [ps, floor(ns) + 1.75], where ps = mesh + mas_len[i] and ns = mesh + mas_len[i+1].
6. Bengali day = floor(nJD − ps) + 1, month = i + 1, year = lastYearCount + 1.

The +1.75 day tolerance helps align month edges with common practice.

Month layout rules

• Fixed 6 rows by 7 columns grid for every month.
• Header shows two Bengali month names by sampling days inside the month.
• Weekday rows include Bengali names and English initials.
• Bengali digits are printed by mapping ASCII 0-9 to U+09E6 to U+09EF.
• The first day of a Bengali month gets a highlight.
• Today is detected in IST and highlighted.

Holidays and observances

• Cell decorations use a simple map of Gregorian date to holiday name.
• If multiple events share a date, they are joined and shown together.
• Ekadashi, Amavasya, and Purnima are shown as per Benimadhav Shil Ful Panjika.

Sourcing policy

• Benimadhav Shil Ful Panjika is the primary source for observances and festivals.
• We review and refresh the dataset each year.
• If other printed sources differ, we keep the Panjika result. We note material differences when useful to users.
• Regional variations exist. The current build targets India.

Corrections

If you find an error, contact us with the date, location, and the relevant Panjika reference if possible. We check the report against the Panjika and correct confirmed issues. Important fixes go into a simple change log.

Caveats

• Tithi labels in the grid use a mean lunar model. They are for context.
• True tithi start and end depend on the longitudes of the Sun and Moon and can vary by place.
• Ekadashi observance rules involve the sunrise tithi and related checks. We follow the Panjika.
• The Bengali date model is practical. Near month boundaries some almanacs may differ.

Contact

For questions or corrections, email us at Coinsmudra@gmail.com.