part-time chair
A chair that is not in use at the particular time it is being spoken about. Examples include folding chairs, reclining chairs, or chairs that one would not necesarily choose to sit in for comfortability.
Were having some family over tonight, can you bring the part-time chairs up from the basement?
time in chair
"Time in Chair" or "T.I.C." means seniority and real-life knowledge a person has most typically at a job.
Even though Bob is my manager and he's got a MBA from Harvard I have more Time in Chair and thus make more money than he does and I also have real experience that he doesn't.