--kerneltz é explicado integralmente na página de manual :
--kerneltz
Use the kernel timezone instead of UTC to determine whether a
packet meets the time regulations.
About kernel timezones: Linux keeps the system time in UTC, and always
does so. On boot, system time is initialized from a referential time
source. Where this time source has no timezone information, such as the
x86 CMOS RTC, UTC will be assumed. If the time source is however not in
UTC, userspace should provide the correct system time and timezone to
the kernel once it has the information.
Local time is a feature on top of the (timezone independent) system
time. Each process has its own idea of local time, specified via the TZ
environment variable. The kernel also has its own timezone offset vari‐
able. The TZ userspace environment variable specifies how the UTC-based
system time is displayed, e.g. when you run date(1), or what you see on
your desktop clock. The TZ string may resolve to different offsets at
different dates, which is what enables the automatic time-jumping in
userspace. when DST changes. The kernel's timezone offset variable is
used when it has to convert between non-UTC sources, such as FAT
filesystems, to UTC (since the latter is what the rest of the system
uses).
The caveat with the kernel timezone is that Linux distributions may
ignore to set the kernel timezone, and instead only set the system
time. Even if a particular distribution does set the timezone at boot,
it is usually does not keep the kernel timezone offset - which is what
changes on DST - up to date. ntpd will not touch the kernel timezone,
so running it will not resolve the issue. As such, one may encounter a
timezone that is always +0000, or one that is wrong half of the time of
the year. As such, using --kerneltz is highly discouraged.
Não vejo nada na página do manual para indicar que --kerneltz é o padrão. Em vez disso, especificou que os tempos dados em --datestart e --datestop serão interpretados como UTC.
O que você realmente deve fazer é definir o relógio do sistema como UTC. Isso deve resolver esse problema, assim como uma variedade de outros problemas que você ainda não conhece.