丙 is the 3rd Heavenly Stem (天干); 辰 is the 5th Earthly Branch (地支, 'dragon'). The Heavenly Stems and Earthly Branches combine to form the 60-year sexagenary cycle (六十甲子) used to date years in classical and traditional Chinese contexts.
history
Famously associated with 苏轼's 1076 Mid-Autumn poem 《水调歌头·明月几时有》, which begins 丙辰中秋，欢饮达旦，大醉，作此篇 ('on the Mid-Autumn of the bingchen year, we drank merrily until dawn; greatly drunk, I wrote this piece').
