Are Time values in music expressed in absolute terms?

Answer 1

Rarely, if ever. Time signatures themselves are relative to the tempo chosen by the performer, and they are also subject to expressive elements like rubato. Other indicators of tempo like Andante or Allegro are always understood to be ranges of tempo and not absolute. Composers of older music may or may not have written some of these indications on the original scores. Even metronome markings (which I sometimes find ridiculously inaccurate in printed scores) may or may not reflect the intentions of the composer. There may be some music marked with a specific metronome value, with instructions to perform the score exactly as indicated. Some music written for special applications like film or other kinds of presentation may have to be performed within fine tolerances for the best fit. Some music by Glass and others may also have more specific tempo requirements.