converting 64 bit millisecond date to 32 bit php

I believe I have a 64 bit date coming in from a website, and I need to decode it on a windows based system.

Example of the timestamp: 1539184503632

When I use:

date("D M j G:i:s T Y", "1539184503632");

it responds with a date in 2038, which leads me to assume it's a 64 bit number processed on a 32 bit system.

I have tried converting it with something like:

$y = $x & 0xffffffff;

but that gives a 2020 date.

Anyone know how to convert this number into a useable format?