Just leave it as is for now, it works.  If it's called in a lot of places we can optimize it with the magic constant. Normally when dealing with time it's best to use these translation functions because you never know which epoch you're going to and from. But in this very specific use case where you're saying the input must be NPTv4 timestamp and the output is POSIX compliant unix timestamp then you can cheat a little and use the constant.