How do I get the server time using REST GET HTTP?
/entity/date_format/long is just giving information on the date format

The easiest and cleanest way to do this is probably to create a simple custom REST resource in a custom module and within the resource you can return the server time using php functions

$message = [
    'time' =&gt; date("Y-m-d H:i:s"),
    'timezone' =&gt; date_default_timezone_get(),
];


You also have to implement CacheableResponseInterface in order to get the current time on every call to the service otherwise you are going to get a cached response.

Here is a working REST API time service module for this which I did just for the sake of this exercise