对的,你把下标用错了而已,应该为days[1] += 1 ,才是润二月的29天。
year = int(input('year:\n'))month = int(input('month:\n'))
day = int(input('day:\n'))
days = [31,28,31,30,31,30,31,31,30,31,30,31]
if year % 400 == 0 or (year % 4 == 0 and year % 100 != 0):
days[1] += 1
now = sum(days[0:month-1])+day
print(now)