No, days are not short in Shangri-la. For its high altitude, it has longer sun hours than many places in China. It is a bit cold in Dec, snow mountain maybe closed. It is more advisable to get there in MArch or April, or around Feb 14, Tibetan New Year. Two days before the new year, it is the Gedong Festival in Zongzanlin Monastery. It is a mysterious religious festival.