datetime must not fall below 1000

What you are doing is turning the tuple into a str and then parsing that into a datetime. But you can also pass your (year, month, day) tuple directly to the datetime constructor:

from datetime import datetime as dt

def days_diff(d1, d2):

    d1 = dt(*d1)
    d2 = dt(*d2)

    return abs((d2 - d1).days)

# result: 3615899
print( days_diff((100,1,1), (9999,12,31)) )```