[twic]

My understanding (i am also not an expert!) is that common NTP implementations will do the slowing down ("slewing") to correct small errors, but will just change the time ("stepping") to correct large errors. It will even do this if that means time going backwards.

Subject to configuration, of course. man ntpd [1] says:

> Sometimes, in particular when ntpd is first started, the error might exceed 128 ms. This may on occasion cause the clock to be set backwards if the local clock time is more than 128 s in the future relative to the server. In some applications, this behavior may be unacceptable. If the -x option is included on the command line, the clock will never be stepped and only slew corrections will be used.

[1] https://linux.die.net/man/8/ntpd

[LukeShu]

timesyncd also does this for "large offsets", but what "large" is is neither configurable nor documented, but in the source:

    /*
     * Maximum delta in seconds which the system clock is gradually adjusted
     * (slewed) to approach the network time. Deltas larger that this are set by
     * letting the system time jump. The kernel's limit for adjtime is 0.5s.
     */
    #define NTP_MAX_ADJUST                  0.4

[fao_]

> neither configurable nor documented, but in the source:

Build it yourself and pass -DNTP_MAX_ADJUST=XX