title: PHP microtime() drift, External NTP service
body: <p>I am looking for a solution for an issue on my cloud-hosted server where the result returned from microtime(true); is very accurate between calls within a few minutes of each other, but drifts by nearly a second over the course of a day or two.  I'm using this result to synchronize between asynchronously connected clients and I need the precision to be accurate to NIST by within &lt;~10ms or so if at all possible.</p>
  
  <p>I know this could be considered off-topic, but the current best option I can think of is to regularly poll some external service and obtain the current NIST time in milliseconds.  Unfortunately I can't seem to find anyone offering this service.  If I need to I'll just set up a second server with reliable time response myself and ping that, but it seems like there has to be someone out there addressing this simple need.  Anybody know of something?</p>