It’s not totally clear to me either, but maybe it’s a 32-bit value treated as a 34-bit value with a precision of 4 seconds (so left-shifted two bits elsewhere),

It’s not totally clear to me either, but maybe it’s a 32-bit value treated as a 34-bit value with a precision of 4 seconds (so left-shifted two bits elsewhere), since the whole term has the least significant two bits cleared with ‘& ~3’. No idea what the (2^31-1) term is doing though.