On a gcc 8.3 on a 32-bit system,
Size of int: 4 bytes
Size of long int: 4 bytes
Size of long long int: 8 bytes
Size of float: 4 bytes
Size of double: 8 bytes
Size of long double: 12 bytes
Size of char: 1 byte

Explicitly cast to (long double) to prevent overflow of the
seconds in a 13-bit week number

Gary pointed out the dangers of using a float to store this
value.

So (long double) is out.

(long long) is 8 bytes, but we can only use 4, because it
is signed.

Use (unsigned long long), which gives us the full 8 bytes.