How to set identical time on server and client?

Question

PHP

I make a request to the server and only GMT is sent back from the server

<?php
echo gmdate("Y-m-d H:i:s");
?>

and on the client, I immediately display the time received from the server and the time on the client

string _servetTime = answer;
string _clientTime = DateTime.UtcNow.ToString("yyyy-MM-dd HH:mm:ss");
serverTime.GetComponent<Text>().text = _servetTime;
clientTime.GetComponent<Text>().text = _clientTime;

the request lasts fractions of seconds (approximately 0.2-0.3 seconds)
the answer is

2020-08-13 11:14:47
2020-08-13 11:14:40

i.e. 7 seconds difference, did a series of tests and the difference was always 6-7 seconds, why is that?
How to synchronize it?

Answer 1

Enable and configure NTP on a single time source.
If the client's OS is not under your control, then you cannot count on the fact that the clocks on the client and server coincide and must figure out how to live with this for your application.