Skip to content
ENExplore Calculators

Lunar Calendar Date Calculator

Work out lunar calendar date instantly with clear inputs, formula shown and shareable results.

About Lunar Calendar Date Calculator

Moon age is the time since a known new moon epoch reduced modulo the synodic month of 29.530588853 days, and illumination follows as one minus the cosine of that phase angle, halved. The mean synodic month is accurate to within a few hours over decades, though the true new moon can differ by up to about half a day because the Moon's orbit is elliptical.

Formulas

Moon age

age = (Julian day - 2451550.09766) mod 29.530588853

Illumination

illuminated fraction = (1 - cos(2 pi x age / synodic month)) / 2

Worked example

With the default values below, Lunar Calendar Date Calculator returns Moon age of 22.339. Change any input above and every figure here is recalculated in your browser.

Inputs used

Date
2026-09-04
Hour of day in UTCaccepts 0–23.99
12

Results

Moon age
22.339 days
Phase
Last quarter
Illuminated fraction
47.97%
Position through the lunar month
0.75646

Step by step

  1. 1Values used
  2. 2Moon age
  3. 3Illumination
  4. 4Moon age= 22.339 days
  5. 5Phase= Last quarter
  6. 6Illuminated fraction= 47.97
  7. 7Position through the lunar month= 0.75646

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Lunar Calendar Date figure worked out?

age = (Julian day - 2451550.09766) mod 29.530588853. Moon age is the time since a known new moon epoch reduced modulo the synodic month of 29.530588853 days, and illumination follows as one minus the cosine of that phase angle, halved.

Why does this matter?

The mean synodic month is accurate to within a few hours over decades, though the true new moon can differ by up to about half a day because the Moon's orbit is elliptical.

What do I need to enter?

Enter date, hour of day in utc. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.

Related Calculators

Lunar Calendar Date Calculator – Calculate Moon age & Phase