I did not remember this one... The first Sequoia demo, after only few days. I also did not realize how impossible is to play in tune with an out of tune piano.
To the OP (in concert key)
This is a Strayhorn tune, so standard harmonic description is a bit stretched.
Bars 1 to 4: Ddmaj going to Eb7, same as "take the a train", but with a secondary dominant (Bb7b5) and a approaching tension note A. Pretty sure that Eb9 implies a #11 (A), even if not in the melody. Very Strayhorn.
Bars 5 to 8: sudden modulation to A, Ab as dominant od Dbmaj. Please note how the similarity in the melodic line with the previous 4bars reigns.
Bars 12 to 14: II V I, II V I, II V (I)
Bars 15 and 16: Strayhorn's chromatic signature, like in "Prelude to a Kiss" or Sophisticated Lady"
Please note the dominant effect od Dmajor on Db major: multiple approach notes from a semitone above, rather than tritone substitution of Ab7.
Pretty much the same on the second half, except that Gb maj.