The Earth Time Standard was a standard of time-measuring based on Earth's orbital period.
Earth years, or, as they were commonly called on most colonial words, T-years (Terran years), were based on Earth's standard year of 365 days. After millions of humans had spread all over the Galaxy in the Diaspora, it was much easier to use this one standard for interstellar time measuring because each world had different days and years. (HH1)
The age of a person was also commonly stated in T-years. (HH2)