Strictly speaking it's only 88 years of M&D (under its own name) and Arriva in Tunbridge Wells, as between 1933 and 1935 the local network was still being operated by Redcar and Autocar. However, the original Autocar was under M&D control from 1 July 1933 (having been transferred from London General Country Services Ltd, or what had previously been East Surrey Traction Co Ltd until 1932), so it's that change that was 90 years ago.
Derek J