Oh! Thanks

I didn't understand. Maybe this would be more understandable?:

 When called with a ‘C-u’ prefix, prompt the user for a starting
    offset or, if there is a timer string at point, this is taken as the
    default, ...

 When called with a ‘C-u’ prefix, there are 2 options: prompt the user for a starting
    offset.  Or, if there is a timer string at point, this is taken as the
    default, ...

Ypo <ypuntot@gmail.com> writes:

In the manual, it says:

C-c C-x 0 (|org-timer-start|)

    ... If there is a timer string at point, this is taken as the
    default, providing a convenient way to restart taking notes after
    a break in the process...
    https://orgmode.org/manual/Timers.html

I am trying it, but the timer starts at 0:00:00

You missed the previous sentence in the manual.

    When called with a ‘C-u’ prefix, prompt the user for a starting
    offset.  If there is a timer string at point, this is taken as the
    default, ...

The value at point is only taken into account with prefix argument.