Technically, a tithi is said to be completed when the phase difference between the positions of Sun & Moon become integral multiple of 12. There are 30 tithis in a Chandra Maas, in which 15 tithis are of Krishna Paksh & 15 are of Shukla Paksh. After an aamavasya, phase difference between Sun & Moon starts increasing & goes to a maximum of 360°. If we divide 360° by 30 then the result comes out to be 12°, which is the value of a tithi.
In Indian Astrology, it is a convention to assume the start of a new tithi at the time of Sun rise irrespective of when the phase difference between Sun & Moon become integral multiple of 12. The tithi lasts till the next Sun rise. It means the tithi present at the time of Sun rise is considered as the tithi till next Sun rise even if the phase difference between the positions of Sun & Moon become integral multiple of 12 at any time after Sun rise.
But due to such an assumption problems arise in determining the dates of various festivals & there is always a state of contradiction among variuos pandits in different parts of the country regarding this. Due to which a festival is sometimes celebrated on two different dates in different parts of the country.
Also due to such a convention, sometimes a tithi gets repeated (which is called as Vridhi tithi) & sometimes it do not comes at all (also called as Kshaya tithi).
between Sun & Moon
|180° + 12°×1|
=360° or 0°