Once interstellar species began to trade with one another, common references of measurement had to be agreed upon. The Standards Convention of Gom met for the equivalent of thirty-two Earth years to decide and agree on these issues. Humans were not part of the convention as they had not yet ventured beyond their own solar system.
Basis: 1 tic = Half-life of Nitrogen-20 (approx 100 Earth milliseconds)
This was the single most contentious issue and consumed over twenty-four [Earth] years of debate. There had to be some common natural basis upon which to base time measurements. Many wanted the basis to be from a resonant frequency of a naturally occurring atom — ideally, hydrogen. At the time of the Convention, several species lacked the high-precision expertise to make this sort of measurement; remaining species were reluctant to share the technology that would allow these measurements.
In the end, it was decided to make the initial standard the half-life of some elemental isotope—either naturally occurring, or not too difficult to synthesize. They would later refine this standard to be based on atomic resonances—calculated in such a way so as to match existing measurements. Nitrogen was chosen for its abundance. The nitrogen-20 isotope was chosen due to it being a manageable frequency for all cultures, even those just emerging technologically.