Why two calendars can give different full moon dates

Two reputable sources give different dates for the same full moon. Before assuming one of them is careless, check three things, because in almost every case the disagreement is explained by one of them and nobody is wrong.

1. Which zone is the date in?

Astronomical data is normally published in UTC. Consumer apps normally convert to your local zone. When the exact moment falls within a few hours of midnight, those two readings land on different calendar dates, and the further you are from UTC the more often it happens. For readers in the Pacific it is routine rather than exceptional.

2. Is one of them naming a traditional month rather than an instant?

Names like the harvest moon or the hunter's moon attach to a particular full moon in a seasonal sequence, and the rules that assign them are conventions rather than physics. Two sources following different conventions can attach the same name to different months entirely. If the disagreement is about the name rather than the date, this is usually why.

3. Is it a calendar with its own month rule?

Lunar and lunisolar calendars begin their months at or near a new moon, but the rule differs: some use the observed first crescent, which depends on where the observer is and on the weather, others use a calculated conjunction in a specified location. A date printed under one rule will not always match one printed under another, and neither is an error within its own system.

The underlying point

A phase is an instant. Everything downstream of it, including which date it belongs to and what it is called, is a local or cultural decision layered on top. When you need to compare sources, compare the instants in UTC rather than the printed dates. If the instants match to the minute, the sources agree, whatever their calendars say.

When it matters in practice

• Planning a photograph. You want the local moonrise on a specific evening, not the abstract date of the peak.
• Tides. Spring tides follow the phase closely, and tide tables are already local, so mixing a UTC phase date with a local tide table can put you a day out.
• Religious observance. Follow the authority that governs it, not an astronomical table, because the rule may depend on observation rather than calculation.

The moon phase tool shows the phase for the date you pick in your own local reading. If you are comparing it against an almanac, convert that almanac's UTC time into your zone first, and the disagreement usually disappears.