The moment of new moon, or that point of time when the longitudes of the Sun and Moon are equal, is called Amavasya (dwelling together
of the sun and moon). A Tithi is the time occupied by the moon in increasing her distance from the sun by 12°. The first Tithi is called Prathami or Pratipad. At
Poornima, the moon and sun are opposite each other, the moon reflecting the sun in full. 30 Tithis constitute a Lunar Month and Amavasya
Prathami (or Pratipad), Dwithiai, Trithiai,Chathurthi, Panchami, Shashti, Saptami, Ashtami, Navami, Dasami, Ekadasi,
Dwadasi, Triodasi, Chaturdasi, Poornami or Ammavasai.
Tithis are Lunar Days. They do not correspond at all to Solar Days. They have nothing to do with the days we experience by the rising and setting suns in our various cities. Our current Western calendar is based on solar days. We are used to thinking in this way. The Moon however does not always rise at night, nor day, nor any time. Actually, the Lunar cycles are much more complex than that. The Sun takes much of our attention because it is so essential for life's operations. And it is so bright, you can't ignore it. The Moon is the opposite, we can fully ignore it, and hardly know where it is ever, unless it makes a bit of a showing as a full Moon or something, then we notice it's beauty. That's about it. But the Moon does have cycles which are very exact, and with modern astronomy we know exactly where it's going to be at all times. In fact, this has been known for a long time- thousands of years- more or less with rough accuracy.
Our current calendar is particularly centered around our reality- our SOLAR DAY and night and SOLAR YEAR and so on.....a lunar calendar takes much more astronomy, math, calculation, astrology and overall intelligence to relate to...but it is much more representative of REPEATING PLANETARY patterns than the rather blunt and dumb solar calendar. The Solar Calendar is like "the calendar for the masses" and the "luni-solar or Vedic is more for real Brahmins, who are simplly more intelligent and trained". But the world uses the solar of course, and that's fine.
So, tithis actually start for the whole world at the exact same moment, but that is on a different day and time on different parts of the country or earth because we all have our clocks set relative to our sunlight day. Then there's "which day is it" so we have invented the international dateline which runs down the Pac. Ocean, and starts the new day in places like Australia- so that's why-In relation to the Earth as a whole, the SUN and MOON CONJUNCTION event in the great sky, which happens every 27 days and starts the new lunar month off, takes place for all of us at the exact same moment(taking Geocentric calculations as usually do).
So then we relate that to local clocks for the benefit of the local clock staring person who wents to know "when it happens for them" so they can follow the changes or whatever. So since "Ekadashi" is actually when the Moon is inbetween 120 and 132 degrees ahead of the Sun, and again at 300 and 312 degrees, therefore these angles actually begin and end roundly 22 hours apart, they last that long, roughly one lunar day, and they start and stop in space at totally odd times in relation to any particular day on earth. So that is why they fall at different clock times in different cities, and this is why Panchangs or Vedic calendars which give the start and stop times of these events must be calculated and printed for a locality, or be written down in Greenwich time or some such standard time frame to be translated by the reader upon need.