Moon Age Calculator (Days Since New Moon)
Find the Moon’s age in days — how long since the last new moon — for any date, with the current phase name and approximate illumination.
How it works
days since a known new moon (2000-01-06 18:14 UTC) = date − reference · moon age = that, taken modulo the synodic month (29.530589 days) · illumination ≈ (1 − cos(2π × age ÷ synodic month)) ÷ 2 · phase from the fraction of the cycle
The Moon’s age is the time elapsed since the last new moon, measured against the synodic month — the 29.53-day cycle from one new moon to the next. Counting the days from a well-known reference new moon and taking the remainder modulo 29.530589 gives the age for any date. Age near 0 or 29.5 is a new moon (dark), near 7.4 the first quarter, near 14.8 the full moon, and near 22.1 the last quarter. The illumination figure is an approximation from a smooth cosine model; because the Moon’s orbit is slightly irregular, a precise ephemeris can differ from this by up to about a day.
Worked example
There was a new moon on September 1, 2026. By September 16, 2026 the Moon is roughly 15 days old — just past full — and close to 100% illuminated, with the next new moon about 14 to 15 days away.
Sources: Lunar phase — Wikipedia · Synodic month — Wikipedia
Frequently asked questions
How accurate is the moon age?
This uses the average synodic month, so it is typically within about a day of the true value. The Moon’s real orbit speeds up and slows down slightly, so for precise astronomy an ephemeris-based calculation is better; for calendars, gardening and general interest this is close enough.
What is the synodic month?
It is the average time from one new moon to the next — about 29.53 days — as seen from Earth. It is longer than the Moon’s 27.3-day orbit because the Earth also moves around the Sun, so the Moon must travel a bit further to line up again.
Why might the phase differ from my calendar?
Phase depends on your time zone and the exact moment of the new moon, and this model uses noon UTC with an average cycle length. Near the instant a phase changes, a local calendar rounded to your own midnight can show the neighboring phase.
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This calculator is for educational purposes. Double-check important results before acting on them.