Per the docs, the rules are complicated, but mostly boil down to "current session's timezone" (which defaults to system timezone):
The current session time zone setting
affects display and storage of time
values that are zone-sensitive. This
includes the values displayed by
functions such as NOW() or CURTIME(),
and values stored in and retrieved
from TIMESTAMP columns. Values for
TIMESTAMP columns are converted from
the current time zone to UTC for
storage, and from UTC to the current
time zone for retrieval.
Of course you can use UTC_TIMESTAMP()
if you need UTC specifically.
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