## C, <strike>35</strike> 34 bytes

    f(n){time(&n);n/=((n/86400)%7-3);}

[Try it online!][1]


  [1]: https://tio.run/##S9ZNT07@/z9NI0@zuiQzN1VDLU/TOk/fVkMjT9/CzMTAQFPVXNHWWNO69n9mXolCbmJmnoYmVzWXAhCkaWhagxkFRUC5NA0ll8yUPPUSheSixOIMPSWgZO1/AA