Party seniority is counted from the day of joining the Party.
Tips
culture
In mainland Chinese usage, 党 by default means the Communist Party (中国共产党), so 党龄 most often refers to CCP membership tenure. Counting starts from the end of the probationary period (转正之日). Often noted in obituaries and biographies as a marker of political seniority.
memory
Same -龄 (líng, 'age / length of') pattern as 工龄 (gōnglíng, years of work), 教龄 (jiàolíng, years of teaching), 军龄 (jūnlíng, years of military service), 学龄 (xuélíng, school age).
