Validating a CPR number
Python (2), 167 156 155 142 141 bytes

from time import*
def f(s):
 try:return 1&gt;sum(int(i)*(ord(j)-48)for i,j in zip("43276504321",s))%11;strptime(s[:6],"%d%m%y")
 except:return 0


tried to do the date validation without the library, but it's 168 bytes:

lambda s:sum(int(i)*(ord(j)-48)for i,j in zip("43276504321",s))%11==0 and int(s[0:2])&lt;([0,32,(29,30)[int(s[4:6])%4==0],32]+[31,32]*2+[32,31]*2+[32]+[0]*99)[int(s[2:4])]


-12 bytes thanks to @GáborFekete

-14 bytes thanks to @RootTwo