drift_test.erl

-module(drift_test).

-export([run/1]).

run(Timeout) ->
    MonoStartTs = erlang:monotonic_time(millisecond),
    OsStartTs = os:system_time(millisecond),
    receive
    after
        Timeout ->
            MonoFinishTs = erlang:monotonic_time(millisecond),
            OsFinishTs = os:system_time(millisecond),
            #{ milliseconds_elapsed =>
               #{ according_to_monotonic_clock => MonoFinishTs - MonoStartTs,
                  according_to_os_clock => OsFinishTs - OsStartTs
                }
             }
    end.

Result of executing it with a 30 seconds timeout while suspending the computer for roughly 20 seconds within the timeout window.

> drift_test:run(30000).
#{milliseconds_elapsed =>
      #{according_to_monotonic_clock => 30091,
        according_to_os_clock => 51650}

OS clock is in the right here - according to my phone timer, execution was blocked for roughly ~50 seconds (even though a 30 seconds timeout was specified and the computer awoke just upon the 30 seconds window coming to a close.)

Tested on OTP 21.3.8.15 and GNU/Linux with a 4.15.0 kernel.

Root cause of the issue appears to be the use of CLOCK_MONOTONIC instead of CLOCK_BOOTTIME when calling clock_gettime - I think.
(This doesn't necessarily apply to other platforms.)

Configuring ERTS with --enable-prefer-elapsed-monotonic-time-during-suspend solved the issue; this makes sense, as the default value of the setting appears to be no.