Purnima is the full-moon tithi that closes the bright Shukla Paksha each lunar month, when the Moon stands 180 degrees from the Sun and its disc appears completely full. It is a traditional day of vrat and worship, most often the Satyanarayan Vrat, and the tithi that carries Guru Purnima, Sharad Purnima and Kartik Purnima. This calendar lists the upcoming Purnima dates for your city, each fixed by the tithi running at your local sunrise. No birth details are needed.
In the Vedic lunar calendar a tithi is the time the Moon takes to gain 12 degrees on the Sun, and Purnima is the 15th and final tithi of the waxing Shukla Paksha. At that point the Moon is 180 degrees from the Sun - directly opposite - so we see its face fully lit. The tithi that follows, Pratipada, opens the waning Krishna Paksha.
Purnima is widely kept as a vrat. Devotees fast through the day and perform the Satyanarayan Puja, traditionally breaking the fast after darshan of the risen full Moon. Because the vrat follows the tithi rather than the clock, this calendar resolves each full-moon tithi against your local sunrise so you know the correct day to observe it.
Every lunar month has one Purnima, and many are named for the festival they carry:
The festival attached to a Purnima does not change how its date is found. Every one of them is fixed by the same rule, the tithi running at your local sunrise, so a named Purnima can fall on different civil dates in two cities exactly as an ordinary one can.
Purnima Vrat is an observance kept through the whole day, so the anchor is sunrise. The rule is stated in Kane, History of Dharmasastra Vol. V Part I, p. 73: religious acts prescribed for the gods by day are begun in the morning even if the tithi is mixed with another on that day, so a day-long observance is governed by the tithi prevailing at sunrise. This calendar takes exactly that day, computed against the true sunrise for your latitude, longitude and time zone.
Occasionally a tithi is vriddha - long enough to cover two consecutive sunrises - and then two civil days both qualify. In that case the calendar lists the earlier day and adds a note naming both dates and saying which was chosen, rather than silently dropping one. The same rule and the same anchor are used for the Amavasya calendar, which is why the two behave alike; the Sankashti and Pradosh calendars use different anchors, because those rites are prescribed for moonrise and for the evening rather than for the day as a whole.
Purnima is a tithi, not a fixed calendar day - it begins and ends at the exact moment the Moon reaches 180 degrees from the Sun. The vrat is kept on the day whose sunrise falls inside that tithi, and sunrise depends on your latitude, longitude and time zone, so two cities can observe it a day apart.
It is a fast observed on the full-moon day, most commonly paired with the Satyanarayan Puja. Observers typically fast through the day and break it after offering worship to the risen full Moon, keeping the vrat for peace, prosperity and the fulfilment of wishes.
No. Purnima is the same full-moon tithi for everyone, so only your city matters - it decides which day's sunrise falls inside the tithi, and therefore which day the vrat is kept. The calendar needs just a location, not a birth date, time or place.
The day whose sunrise falls inside the full-moon tithi. When the tithi is long enough to cover two consecutive sunrises, the earlier day is listed and the entry carries a note naming both dates, so you can see that the case arose rather than having to guess why one was chosen.
They are related but not identical in use. The astronomical full moon is a single instant, when the Moon reaches 180 degrees of elongation from the Sun. Purnima is the whole tithi that ends at that instant, and the vrat is kept on the civil day whose sunrise falls inside it - so the fast can be on the day before or after the clock time of the full moon itself.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.