Zugzwang In An Opposite-Coloured Bishop Ending

TODAY sees the second double-rounder at Malta, and this morning I won thanks to my opponent blundering horribly in an ending of opposite-coloured bishops.
Such endings are notoriously drawish, especially when material is equal (!), but a modicum of care is required.

White to make his 53rd move in the round-six game Walter Cools (1766) - Spanton (1940)

*****
*****
*****
*****
*****
53.Kd3??
White had a simple draw after 53.Kd2.
53...Kc1
But now White is in zugzwang, and so was obliged to resign.