On the website of the U.S. Naval Observatory (USNO) the following definition can be found for the phases of the moon: "Technically, the phases New Moon, First Quarter, Full Moon, and Last Quarter are defined to occur when the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 0, 90, 180, and 270 degrees, respectively. These definitions are used when the dates and times of the phases are computed for almanacs, calendars, etc. Because the difference between the ecliptic longitudes of the Moon and Sun is a monotonically and rapidly increasing quantity, the dates and times of the phases of the Moon computed this way are instantaneous and well defined."
However, it is more logical to define that full and new moon occur when the lunar distance of the sun reaches a maximum and a minimum value, respectively. For the time and date of the moon phases on this page this definition was used. As a consequence, for full and new moon the time on this page may differ from the official time by up to 30 minutes or slightly more. The reason for this difference is that the orbit of the moon is slightly inclined relative to the ecliptic plane.
If a solar or lunar eclipse occurs, the maximum of the eclipse occurs exactly at the moment when the lunar distance of the sun takes on its maximum (eclipse of the moon) or minimum (eclipse of the sun) value. Therefore, if an eclipse occurs, the time on this page is the time of the maximum of the eclipse. For full moon, if the lunar distance of the sun differs from 180° by less than the Horizontal Parallax (HP) of the moon, there is a total lunar eclipse. Since the HP of the moon is about 1°, a total lunar eclipse occurs when the lunar distance of the sun is larger than 179°. For new moon, if the lunar distance of the sun is less than the sum of the semi-diameter (SD) of the sun and the moon, there is a total solar eclipse. Since the SD of both sun and moon is about 15', a total solar eclipse occurs when the lunar distance of the sun is less than 30'.
Data was generated using Skyfield. From NASA's Navigation and Ancillary Information Facility (NAIF) the following ephemerides were used: de421.bsp, jup310.bsp, sat375.bsp.