Tithi is the lunar day. It is not the same as the date on a wall calendar. It is measured from the angle between the Sun and the Moon. When that angle grows by twelve degrees, one tithi gives way to the next. There are thirty tithis in a lunar month: fifteen in Shukla paksha (the waxing fortnight) and fifteen in Krishna paksha (the waning fortnight). Pratipada, Dwitiya, through Purnima or Amavasya are the familiar names.
Because the Moon does not keep civil midnight, a tithi can begin or end at 10:40 in the morning or 3:15 at night. That is why a panchang lists both the tithi at sunrise and the end time. Two cities in Rajasthan usually share the same tithi name on a given morning, but the end clock can differ by a few minutes.
Festivals often follow tithi, not the Gregorian date. Ekadashi, Purnima, and Amavasya are tithi observances. City sunrise still matters for the local end time if you are deciding which calendar day to keep.
On Apna Panchang the hero card shows today's tithi for your selected city. Open a full day page for yoga, karana, and nakshatra next to it. If you are comparing two towns for travel, switch the city in the header rather than trusting a Jaipur newspaper time in Bikaner.
